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

Answer 1

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.

Answer 2

eat a meal with him. any meal. a man with bad manners will chew with his mouth open. talk with food in his mouth. not use a napkin. reach across the table for things instead of asking. he'll rest his elbows on the table. he'll belch or use profanity at the table. and when the meal is over and your cleaning up, he should offer to help clean up and thank you sincerely for the wonderful meal