As a DM, when players are interacting with an NPC and are considering the posibility of murdering them, they ask me subtle questions like "how big is he?" "does his sword seem to defy gravity or slice through the air faster/smoother than normal, or is it like a big heavy encumbrance to him?" as a way of figuring out what level the NPC is. Or, really, how close to their own level he is. Generally I'll answer with things like "he looks like a formidable challenge" or "he could definitely carry himself in a big bar brawl" or "the way he just ripped that tree out of hteground, used it to make a golfshot with the cart on that bridge, then javelin-tossed it into the next ravine, I'd say he can take you on and win without even realizing he was under attack."
However, as a player, the group I am with is being forced to turn over our newly found treasure (a magic device. what it does, we don't know yet, but someone offered us 800gp to get it) to YourStandardAdventuringGroup. It's a halfling thief, a human sorcerer, dwarf fighter, etc etc. While we have made a diplomatic solution to the situation (at least, a partial solution, we're still minus the payment promised by contract), we are considering combat to resolve the rest of the situation, but our DM absolutely refuses to give us any indication at all of what these characters levels are. "How big does he look?" "Dwarf sized." "Gee, really, the dwarf is dwarf size? Alright, that helped us out a lot, thanks!"
What kind of skill or ability can be used to determine another characters level in relation to your own? I don't think whatever it is should be charisma based (like diplomacy) because even noncharasmatic people in a bar can take aquick glance around and see who they can take on and who they shouldn't mess with. Heck, even stupid people usually can, tho sometimes they don't really head the warnings from themselves ... so I'm tihnking more of a wisdom based something instead.