Technically an Eidolon is 'just an outsider'.
But most eidolons are unique entities and/or members of an extremely obscure outsider race, depending on your in game cosmology. So, a schooled individual (ie, someone with ranks in Knowledge: Planes) is likely to be capable of guessing that an eidolon is an eidolon when then realize they've never actually heard of a six legged horse with a scorpion's tail that breathes fire before.
Also, synthesist's eidolons always appear to be made out of some form of ectoplasmic stuff.
