If you go to another plane and one of the native creatures tries to identify a material plane PC (assuming human for simplicity), what kind of knowledge check is it?
Per the Knowledge rules for identifying creatures, it would normally be a Knowledge (Local) check against a DC† of 10 + CR of the humanoid being observed. Since that table is material-plane-centric though, it would be reasonable to think of a human on its non-native plane — being checked out out by a native of that plane — as more like an outsider for Knowledge purposes and use the skill used for the Outsider type instead of the Humanoid type, which is Knowledge (Planes). Since humans come from a different plane than the one rolling the check, that would make sense anyway.
Since the check is being used to merely identify the creature's species, not to ascertain that exact individual's capabilities, you would probably want to use the CR of a “basic” or “typical” human (most humans are CR 1) rather than try to calculate the specific CR of that individual with their classes. (If the creature wants to know something more than “what is that thing?” then use their actual CR, of course.) That makes it simpler too: CR 1 gives a Knowledge DC 11. So basically, humans as a species are pretty widely known on the planes, to those who have some education. Considering how much humanity magically meddles with beings from the planes, that seems right.
† DC 10 + CR assumes humans are neither rare nor common on that plane already. Per the Monster Lore rules, this should be lower (DC 5 + CR) on planes where humans are relatively common, and higher (DC 15 + CR) on planes where humans rarely appear.