How do you tactfully ask a guy if he is gay? If he isn't I don't want to offend him. ?

Just say, No disrespect at all, but are you gay?

I've been asked this a few times, I wasn't offended at all

Be open from the first with him, maybe something like

"Hi. I am sure you realize that I am gay and I was wondering if you would like to go out Saturday, if not I am fine with that could we still be friends."

There is no real tactful way - either the person is going to be understanding and respectful or they are going to get pissed and get angry.

Make a joke out of it. Just say, "Hey, you wanna run by the morgue and sip a cold one?"

Just kidding.

"Hi, would you like going out on date Saturday night?

I've got two concert tickets!"