TL;DR: It would be realistic for a PC to be able to evaluate how mechanically strong their opponent is, but I couldn’t find any RAW for this. Am I missing something?
Certain skill checks in DnD, like Nature or Religion, allow PCs to learn bits of lore or narrative information about their opponents. Depending on the check result, you can learn about their position in the world, strengths and weaknesses, common knowledge, even types of attack.
However, from what I understand (correct me if I’m wrong), such a check can’t tell the players any mechanical information about the creature - their HP, AC, CR, etc. But in a game as combat-oriented as DnD, knowing an opponent’s stats, or at least how they relate to yours, is vital to make an informed decision on how to proceed with this interaction - behave recklessly, cautiously, etc.
Imo, it’s not unreasonable to wish to be able to evaluate your opponent. After all, IRL by looking at someone you can get a general understanding of how dangerous they’d be in a fight. In some marginal cases looks can be deceiving, but on average - you’d know who can kick whose ass.
In DnD, however, where we have CR5 trolls and CR16 snake-girls (Marilith), PCs can’t base their decisions on the looks of the opponent. But, realistically speaking, living in this world they should know how to evaluate someone’s relative strength.
So, how can I let my players know the power gap between them and the other creatures?