Recently in a session, I almost caused our party to fight a lich at level 5. For D&D 5e, a lich has a CR of 21. There were a few clues to his strength that I missed, such as being locked in a room intentionally with a relatively powerful spell we managed to dispel, that he threatened to fill the room with gas (lich power), and that he was creating spell scrolls.
My party and I are relatively new to this. My DM told us he was basically an undead guy, but I had fought them before and wasn't much impressed. We later found out he was a lich after the session and my DM told us the various ways in which he would've fled while almost killing us all in the process. This was quite a shock and my party was a little concerned with my aggressiveness towards this random NPC that told us to leave rather than outright attacking us.
What are some ways my PC can find out how powerful an NPC is without prior knowledge of his CR and without the DM telling us explicitly what the creature is? I intentionally don't read the Monster Manual so that my character will be as clueless as I am in real life.
I generalized what the DM had described the Lich as because I felt it wasn't important, and I still feel that way because this is a player issue, not a DM issue. However, since a lot of you are commenting on the description, I feel I should clear the air a bit. Here is the context as best I can remember it.
We dispelled a magical seal on a door and busted into a laboratory with ancient tombs and alchemical stuff everywhere. There was a man at the end of a long room and he took no notice of our entrance, though he knew we were there. He was busy reading or writing something, it was hard to see. My character has a strong affinity for books, the arcane, and especially forbidden knowledge. I ran up to him and upon getting closer, noticed that his flesh was falling off of his body in a very undead sort of way. "Leave now or I'll flood this entire room with gas", said the man in an intimidating tone (effort #1 of my DM to discourage me as most characters are not role-played this way). Knowing nothing of Liches or undead that can speak in general, I, the player, kept forward, asking questions about who he was and what he was doing. As I came closer I noticed that he was writing a scroll of invisibility. I also then noticed, as he turned to face me, that his eyes were white and had a dead look in them (undead clue #2). I, the player, assumed some quick get-away with the scroll and perhaps some lever I could not see that would deploy the gas. I almost cast a spell on him to keep him there, but decided to ask him more questions, testing the limits of his patience.
It was at this point he told us he was a lich and he urged us to leave him be. Then we made some deals, we left, and the session ended. I do not feel my DM was inadequate in this matter, so much as I did not push him to find out who or what this man was initially and that we as a group play in a way that forces us to deal with consequences. He is not there as a safety net, and we as a group like it that way.