It's almost balanced, but...
Let's look at the scores for a Kuo-Toa in the Monster Manual.
STR 13 (+1)
DEX 10 (+0)
CON 11 (+0)
INT 11 (+0)
WIS 10 (+0)
CHA 8 (-1)
Their highest ability score is actually Strength, which is neither of the ability score increases covered. If I had made this, I would have made the increases +1 Strength and +1 Constitution.
I also would have given this race a swim speed of 30 ft., as the race is literally a fish, and it is in the stat block. Most of these features are balanced out by Sunlight Sensitivity, a great, big, debuff. If anything, removing the Otherworldly Perception would make the race underpowered.
Surprisingly, this race would fit best in a melee class if the stat block was followed (barring Sunlight Sensitivity), but it seems that the creator attempted to shoehorn the monster into a spellcasting class. Otherwise, everything seems legit, as I cannot find a source that argues against it.
On the fact-based side of things, let us look at the traits from a user's standpoint.
Languages: Undercommon is not the most useful language, but good for campaigns that deal with drow and the like a lot.
Otherworldly Perception: As enemies on the Ethereal plane are rare, I'm going to ignore that. Seeing through invisibility, however, is powerful. The hole in this is that the Kuo-Toa cannot see the creature (and hence does not know whether it is an invisible Duergar or an invisible Archmage), and only knows the creature's location if it moves. This is akin to the first-level spell Detect Magic:
An aura forms around objects or creatures you can see, indicating that it is magical. It must be within 30 feet. You also learn its school of magic, if it has one.
- Sunlight Sensitivity The reason this race is balanced lies in this trait. Melee classes are killed by this feature, and (most) spellcasters don't like getting within 30 feet of a creature just to pinpoint its position. Disadvantage on Perception is hard to ignore:
"Yes! I rolled an 17 for Perception! What do I see? Oh, wait, disadvantage...nat2."
Amphibious: Saves you a few gold pieces on Potions of Water Breathing.
Slippery: Great for melee classes, especially Monks, who don't usually rely on Strength.
The reason that this is almost balanced is its way of not fitting into any particular class well (if it had the correct ABC). If you are a wizard, chances are you'll be too far away to use Otherworldly Perception. If you are a Fighter, disadvantage on attack rolls is TERRIBLE. For WIS based classes...it may fit, but they really shouldn't have these ability score increases that make them attractive for those classes. Wisdom is NOT the Kuo-Toa's strength; Strength is their strength.
Simply allowing more arms is overpowered
Allowing a PC to use a two-handed weapon with a shield is overpowered indeed. The game wasn't created with that in mind: either your character use a two-handed weapon to deal more damage or use a shield with a one-handed weapon, trading-off the higher damage for better protection.
By simply allowing a PC to have four-arms, without limiting what it can do with those will surely break the game. The PC will be able to ignore a lot of "exclusive options", trade-offs it can simply avoid; the two-handed weapon with a shield is just one of them.
Spellcasting with a sword and shield (without War Caster), wielding a shield with a spellcasting focus and dual-wielding, using a hand crossbow with an shield, and any other situations that can't happen when you only have two arms are some of the trade-offs a four-armed creature can avoid.
So, how to balance it?
If I were to create a Thri-Kreen race, I would limit what it can do with those extra arms. A good starting point is to limit what interactions can be done with those extra arms. The Trunk feature from the Loxodon (Guildmaster's Guide to Ravnica, page 18) gives us a good starting point:
You can grasp things with your trunk, and you can use it as a snorkel. It has a reach of 5 feet, and it can lift a number of pounds equal to five times your Strength score. You can use it to do the following simple tasks: lift, drop, hold, push, or pull an object or a creature; open or close a door or a container; grapple someone; or make an unarmed strike. Your DM might allow other simple tasks to be added to that list of options.
Your trunk can't wield weapons or shields or do anything that requires manual precision, such as using tools or magic items or performing the somatic components of a spell.
Of course, the thri-kreen extra arms aren't like a trunk, so it needs tweaking. I would reduce the number of pounds it can lift, and well, a thri-kreen can't use arms as a snorkel. I would also remove its ability to grapple or interact with a creature; their extra hands are small. Instead, I would allow them to have "one additional interaction with an object or environment, for free, during your turns".
The most important limitations of extra arms are in the second paragraph. I would never touch those, because that's where the majority of problems lies with extra hands.
As Matthieu M. pointed out in the comments, a good explanation for why shouldn't a Thri-kreen extra arms be limited, is that the creature's brain is limited, and can't coordinate so many tasks with the required precision.
Best Answer
Some background: I love to homebrew things. I'm constantly creating new stuff for fun. These are the most important things I've learned in the process.
Compare it to official race/classes
The first (and most important) step is to compare it to official and published race/classes. Usually the PHB is enough. You can either create a rating system of your own or use a well acepted rating system from the internet, e.g. This homebrewing race guide. Note that it only works if you assume the original devs did a decent job in balancing whatever they published. For example, in my opinion, this is not true for feats in 5e, which are heavily disbalanced. It works for races and classes though.
Usually, you should be creating something "close" to the original features. Do you have something that deals elemental damage in an area? Compare it to Dragonborn's Breath. Do you have a flying race? Compare it to Aarakocra. If you stray too much from the original books, you might find yourself in a difficult position where only experienced DMs should be. Do you have a heavily martial type class? Compare it to Barbarians or Fighters, etc.
The Wizard's official site also has some insights on Modifying Classes and the DMG has some insight on modifying races. Although the Unearthed Arcana (UA) is not playtested and well balanced, it might serve as a good start as well. Some of the UA races/classes have feedback from the community. Check them.
Playtest it
You choose how. Find a table of friends that are fine with you using them as lab rats. Search for an online playtest group. Do whatever you are able to. But most importantly, playtest it. Nobody has crystal balls. Nobody knows if something is going to be balanced or not. The official release of 5e had lots of playtests before actual release. Sorry, but you're not better than an entire devs' team dedicated solely to game developing and balancing. You will screw things up and make OP and UP homebrews that will completely break the game or suck, if you try enough times. That's OK. That's why you try it out: to check if it's okay. If it is not, remake it taking into account whatever you learned from your tests.
Compare it to your previous experiences
Well, the whole point of playtesting is to learn. It is okay to make mistakes. But you need to learn from it. Oh, 10d6 was too much damage for a race feature that you get at 1st level? Who would have thought?! Okay, let's tone it down to 1d6 and try again. Oh, I'm trying to create a flying race. Hmm, Aarakocra and that Winged Elf that I tried out seemed OP for Tier 1, maybe I should give the wings only at 5th level? That might be better. Anyway, learn from your past mistakes and even from published mistakes.
Get Feedback
After you have homebrew'd your thing, you can always post it here and get some advice, or, if you feel more like a idea generation and feedback process, on usual RPG discussion forums. Obviously, the feedback from people that actually played with that (i.e. the people which you playtested with) are the most important ones, but the internet has some good theory crafters around.
Bonus
And honestly, that's all I can say. Other than that, you might find specific advice if you google your problem or query search here at RPG.SE. For example, if you google "Homebrew race guide 5e" you will find the guide I linked to. If you google "Homebrew class guide 5e" you will find some decent guidelines for classes (granted, balancing classes is way harder than races, reason this question might get closed).
Disadvantages vs Advantages
This edition, opposed to, for example, 3.5e, went for a more "only bonus" path. Essentially, this means you don't lose things by getting something new. You don't lose your race bonuses when you choose a subrace. Most feats are positive-only. The few trade-off things, like Sharpshooter, are optional (i.e. it's not a permanent debuff on attack bonus). You don't lose ASI with most playable races (exception for Monstrous Races in VGM, which are explicitly stated as not balanced and that you should take care using them). Races have resistances, not vulnerabilities. Classes have, at most, restrictions when they can use their features, not actual impairments, the only exception I'm aware being the Metal restriction for Druids, which is not exactly huge (in fact, it has been stated in the Compendium that this restriction is thematic and linked to D&D history, rather than balance, and removing this restriction is ''not going to break anything in the game system'').
This is a way of thinking seen in many online games, where the devs prefer to buff things rather than nerf. Basically, we feel happier getting better things. We actually feel happier getting a +1 rather than a +7/-6.
By this philosophy alone, trying to balance a huge buff with another huge nerf is usually a bad idea in this edition. If that's not enough for you (and shouldn't be), as I mention in this meta, most features are situational. When you try to balance a huge bonus with a huge impairment, there are two most likely scenarios, both bad for everyone: