How do you know when a boy doesn't have manners?

The best way to know is to interact with him yourself.

If he uses foul language (e.g. profanity, vulgarities, etc), makes rude gestures in body language, and speaks of others behind their backs in a not so nice manner then he probably doesn't have any manners of his own, or at the very least doesn't show them when necessary.

Be sure you get to know him to some degree before making a complete judgment call on his mannerism.