This comment from another question has me worried:
I am curious if you had a specific reason for allowing the player to find a +2 Shield given their level.
This has me worried because I just allowed a player's level 2 Ranger to purchase a wooden shield (+2 AC) from the PHB's equipment list.
It seemed reasonable at the time: the PC was talking to an armorsmith in his shop and had enough money to pay the listed cost for a shield. To me, a novice GM, +2 AC didn't seem game-breaking at all, however the linked question has highlighted that accidentally giving players potentially unbalanacing items is a possibility.
The shield hasn't been too much of an issue so far, but I am worried I could make similar mistakes which could be more difficult to fix. I've not had any specific problems as of yet, but it seems like I could quite easily give them what I perceive to be "a standard piece of equipment" again but it turns out to be A Big Deal.
Given my lack of experience with RPGs and as a DM, how can I tell ahead of time whether an item is considered "overpowered" for a particular PC level, or may cause difficulties for me further down the line?
It seems that there are enough questions about GMs being concerned about overly powerful PCs[1],[2],[3],[4] to warrant this question. The fact that questions exist like "abnormally high AC" is worrying and makes me unsure about what I don't know about equipment and balance.
Best Answer
Tables in the DMG can help
As you say, there's no substitute for knowing the power of magic items in any given system. Indeed, even the DMG itself has some wonky valuations on some of its magic items.
That being said, there are several tables in the DMG that I often refer to to get a sense of the power of specific magic items.
The first is on DMG 38, which describes the starting equipment for PCs at different levels. That table should show you what the designers considered to be appropriate magic item loadouts for characters at those levels. This table is mostly useful for knowing how many magic items a PC should have, though you should keep in mind that those are only starting values, and characters are expected to get more magic items throughout a campaign.
The second is on DMG 135, which compares the rarities of magic items to character levels. This is useful for knowing how powerful a magic item should be, and at what level characters should be getting those items. You can combine this with the table on DMG 285, which compares the rarities of magic items to the spell level of the spells they recreate.
Not every magic item duplicates a spell effect, however. There's not much help on determining what the rarity of a magic item should be--the only way to know that is to read through the existing magic item list to get a sense of how strong an item should be at a particular rarity.
An example
According to the DMG, a +2 shield is rare. Using the Magic Item Rarity table on DMG 135, we can see that it's appropriate for a character of level 5 or higher. Looking at the table on DMG 38, in the "High Magic Campaign" section, we see that characters starting at levels 5-10 don't have any rare magic items, but characters starting at levels 11-16 have one. This suggests that the +2 shield should probably be the only rare magic item that a character should have, at least until much higher levels.
Note that a +2 shield gives a total of +4 to AC, because ordinary nonmagical shields give +2 to AC by themselves. The nomenclature is such that any modifications (+1, +2) are magic modifications beyond the mundane properties of that item. Therefore, it looks like the shield you gave to that rogue is just a mundane shield, which is balanced by other mechanics (can't use a bow, can't use two handed weapons, etc.).
Being OP isn't a big deal
I like to give far more magic items than these guidelines suggest, mostly because magic items are a lot of fun. Honestly, I don't think that it's too imbalancing, unless you give one character a lot more magic items than the others. After all, you can always increase the difficulty of your encounters to compensate for the increased power that your PCs will have. Plus, I think that many players enjoy steamrolling a few encounters with their magic items, so even if you misjudge a few encounters, it's not a big deal.