The rules on the Knowledge skill are helpful in explaining what people might be expected to know and identify.
Most people won't have these skills, and will thus only be able to make a 10 at best.
You cannot make an untrained Knowledge check with a DC higher than 10.
This means identifying a creature isn't going to work out for most people, except for very common monsters.
You can use this skill to identify monsters and their special powers or vulnerabilities. In general, the DC of such a check equals 10 + the monster’s CR. For common monsters, such as goblins, the DC of this check equals 5 + the monster’s CR. For particularly rare monsters, such as the tarrasque, the DC of this check equals 15 + the monster’s CR or more.
Even a local low level adept or cleric would only know stuff about low HD critters.
As @MACN points out, what monsters are "common" would be dependent on where you are - in Geb undead are, in Falcon's Hollow kobolds are, et al. Drow are not known anywhere (see Second Darkness - they are mostly-unknown creatures of myth).
Even "Identify a creature's ethnicity or accent" is DC 10, so for common PC races a particularly dumb person might mix up a gnome and a halfling, or a Chelaxian and a Taldan.
Identifying an eidolon is not "really" a whatever it looks like would take Knowledge: Arcana or Spellcraft. The summoner/eidolon's mark would certainly link them, though "that guy is under the succubus' domination!!!" is as valid a guess to the untutored.