There is no absolute way to tell, no spell that says "he's level 10/10 HD." You have to go with context clues and observation instead. That leads us to a two-part answer.

DM Description and Observation

Observe more closely and the DM should be more forthcoming with details. In many cases, higher level NPCs/monsters look different - think World of Warcraft, you can tell a high level person because of their crazy pimp looking gear and purple energy coming off them and stuff. Maybe he has ioun stones whizzing about his head, etc. Judicious use of detect magic, detect evil, etc. can tell you strength of auras. A DM should also think through setting up context clues per How can DMs effectively telegraph specific dangers in D&D? - like maybe someone powerful comes to mess with him and gets disintegrated while he's chatting amiably with the PCs.

Many high level characters are obvious - a gnome in a robe of the archmagi riding a dinosaur with all kinds of magical protection glimmering around him is obviously bad news. If all your gear is platinum and adamantite it means you're bad ass enough to keep a hold of it from random bandits and/or adventurers. Your DM should be conveying this detail, as long as you pause to take a look.

In some cases (monsters, undead) skill use may let you get a better read on what it is you're facing. In 3.X this was sadly routine (I will roll and you will tell me exactly what this is), in 5e this isn't as guaranteed a method of knowing exactly what's up and there's more DM discretion, but at least asking for a roll and getting hints might help.

It definitely sounds like your DM needs to be thinking through this more and giving more context. "Hey this dead guy looks a lot like that statue of Archmage Xulibraz back at the Acadamae..."

Play Smart

In some cases, a high level person isn't going to look/detect different from a low level person. This should be more rare unless your DM is a dink, though there are exceptions. A high level monk in rags looks like anyone else and a king might have super pimp gear despite being level 1 just because of daddy's money. Now, if you see the monk fight it should be like watching Jet Li and you should get the clear impression he could snap your neck and isn't some random yokel. But this is where the onus shifts to you. If you decide to just roll up on someone without knowing much about them, there's a good chance you're going to get owned one day. You have to be smart.

Observe folks. Ideally, use time prior to the actual encounter. If you know about them way ahead of time then there's divination spells or sage research or "asking around on the street." See if you can observe them fight someone or cast a spell or use a skill to where you would see "holy crap they are about 500% better at that than I am." Have a backup plan. If you just kick down doors and attack people, 1 in 10 times you're going to bite off more than you can chew and you'll need a fallback. That floating thing could be a beholder or a gas spore. That undead could be a zombie or a lich. Knowing that there is that range of foes, it's up to you to decide whether you want to be all reckless and maybe die one day or whether you want to be super cautious with everything or where you want to fall in between - just like real life.

Conclusion

You have just learned a valuable lesson, and learned it without dying! Congratulations. The world is dangerous and it's not always neatly labeled or level appropriate. Let this inspire you to think about how to suss out your foes both from afar and when in direct contact with them. It'll keep you alive more than a fistful of plusses.

Let me start off by saying that Curse of Strahd certainly has some very challenging encounters, many of which can be stumbled into at points when the party is woefully under-levelled. I've only run it once so far, but that did end in a TPK.

In Vallaki, the party has the opportunity to run into a nest of vampire spawn whilst investigating a coffin maker's shop. The coffin maker (at least under my interpretation) is being kept as an unwilling servant/hostage of these vampires. I tried to give as many hints as possible that the coffin maker was scared of something in the shop - he was glancing over his shoulder, refused to let them in, would only speak in a whisper and asked the party to meet him later 'somewhere safe' suggesting the inn. Unfortunately the party didn't take the hint. Instead, they separated, two distracted the coffin maker, whilst the remainder broke into his house. I tried to give further hints - letting them spot many footsteps around large body sized crates in the dust of the storage room. I even pointed out that the cleric in the party felt something deeply disturbing from the room. Irrespective, he cracked open the crates, triggered an enormous fight and ended up dying.

It's important to note, however, that this type of danger is a common feature of sandbox campaigns. On top of that, this is a horror campaign - it is not unreasonable that if the characters poke around the dark corners of Barovia whilst uninformed of what lies within, they may end up angering something much more powerful than themselves. I personally feel that without such a threat, the player paranoia and tension necessary to maintain an effective horror theme can end up being lost or diminished.

Players should be made aware of this when starting the campaign and they should note that often discretion will be the better part of valor. It also behooves the DM to provide more narrative warning in such cases - signs of danger, direct warnings from NPCs, and (for when it all goes awry) avenues of escape. Even with this, however, encounters can and should have the potential to be fatal. At that point it is down to each DM how such an encounter plays out - nothing precludes you from granting strokes of luck, fudging rolls, or throwing in timely ally arrivals when things go south. Only you know your players and the game you are running, hence it's up to you and what you most feel makes for a fun game around your table.

With regards to your specific examples:

Death House - this is a hard starting adventure, but one that I found to really set the scene and vibe of the campaign. The shadows at the statue, the two ghasts, and the shambling mound stand out as all potentially being fatal encounters. The shadows, however, are extremely avoidable and are essentially a consequence of a lack of caution. Much of their threat depends on the party composition and their ability to output magical damage. Contrary to the book I would probably not have the shadows pursue the party out of the room, and may have them vanish if (for example) the orb was returned. The ghasts and the mound are tough, but certainly doable if the characters play smart.

and

Bonegrinder - I feel that warnings discouraging the party from entering too early are key to this adventure. The Vistani at Tser Pool may have warned the party away from there, knowing their destination and based upon what is written on pg. 28. Additionally, the raven should be extremely vocal in trying to ward off the players. Remember, the raven will be in league with the wereravens of Vallaki and essentially a force of good. Hence, assuming the party did head in and end up in fight against the three hags, having the raven summon help in the form of a swarm or a wereraven may be one solution to avoiding a TPK.