Short of asking him or seeing him blow a pal at Thanksgiving dinner you really are stuck in a game of guessing. Maybe he fits the mold of a classic gay guy or maybe not. Looks, sounds, styles, mannerisms, and the rest can be misleading. Not only that, they probably only help perpetuate stereotypes which feels yucky for everyone. So I guess you have to bite the big one and either ask him outright or dig for some hard evidence. That said, maybe you don't know because he wants to keep his sexual affairs private. Maybe he makes his private life private on purpose. Not to hide his homosexuality necessarily, but because - I don't know - your family is nutty and he likes separating the two? Or the last time he brought his girlfriend over your other uncle hit on her?
I say let the mystery reveal itself. When you are least expecting it he might drop a clear lead in for you to pounce on; a line so obvious you have no choice but to pass the stuffing and say "So how long have you and Richard been dating?"
How can you tell if your uncle is gay?