If the conversation was the only thing that clued you in to the fact that he was gay, I would go ahead and date him. It's usually pretty obvious when a man is gay, at least to me. And if he's attractive, this dude you had the conversation with could have been interested, or hoping that he was gay, and was trying to keep you from being interested. That may be kind of a stretch, but it's possible.
Just check him out from a distance for a while. I'm not trying to stereotype or anything, but what kind of clothes does he wear? Does he always dress nice? What kind of mannerisms does he use when he talks? What does his actual voice sound like? Does he "hang out" with a lot of women? If he does hang out with dudes, are they gay?
It can be tricky to determine for yourself whether someone is gay or not if you're not used to doing it. Unless you're really attracted to this guy, I wouldn't bother with it.