I'm a Lawful Evil (evil like greedy and selfish) warlock with an Imp familiar (through Obtain and Improved familiar feats) in a party with a Good druid. The druid does not have Knowledge Religion or Planes, so he should have no idea what the Imp familiar actually is.
I cast Eldritch Blast at an enemy and that druid succeeds his Spellcraft check and knows what spell I cast. Does that information allow him to claim that I am warlock and throw accusations at me of me being evil incarnate, etc.?
It feels pretty strong for one unrelated skill check to completely reveal class features of other characters. I remember I saw Knowledge checks for some prestige classes, like Knowledge Planes for Hellfire warlocks, to know something about them — not including class features, etc. of course. But I have not found anything like this for the warlock class or a few others base classes.
My warlock pretends to be a mighty wizard who just doesn't need to study books to remember his arcane arts (sorcerer would probably be even better) and I have an insane Bluff bonus (though as a rule we don't really use skills like diplomacy, etc. within the party, since that would make no sense; but Spellcraft check to know what spell is cast seems legit).