However, All of that information would require you have something to go on for him being a Binder. i.e. that would be the checks if someone said "Hi! I'm Gorgoth, the Binder", and you wanted to know what that meant.
Right now, you've see the Ranger-type guy pull a bird out of nowhere. He has deliberately not let you see any of his signs, and his (Su) abilities don't show anything other than their effects.
You do not know that the bird is a magical effect, and even if you used 'Detect X' on it right now, the best you could get from it would be that it is basically the same thing as a Spider Hand spell, with some changes due to type and school.
His character is deliberately keeping what his training was and what his abilities are (metagame: his class) a secret. And he's doing a damn good job of it. It would be a crappy DM move for me to let someone else undo that with merely a Knowledge check without something to go on.