It's common for players to assume that opponents are always “level appropriate.” However, mixed level modules change this situation. Recent modules like Tomb of Annihilation can have PCs stepping into areas far beyond their skill.
This raises the question though: what in-game ways to the PCs have to estimate the power and capabilities of their opponents? If the PCs are an inexperienced level 1 party and meet a CR4 opponent where often one strike could kill a PC, not knowing this could be a total party kill.
(For the purpose of focusing this question, please ignore experienced players who know the contents of monster manual or who have gained out-of-character information they can use for judging enemies. Besides, even that often doesn't work for NPCs with class levels!)
I'm not looking for how the DM can hint at NPC/monster power — those are DM techniques. I'm looking specifically for what mechanics and rules players can use to have their PCs gauge the power of an opponent — whether that might be hit points, CR level, stats like strength, talents or magical abilities. How can PCs judge the opposition's strength?