Why would you want to do this? It's really none of your business what they think or feel. If this person is a friend as you say, you should already have some idea of what they are like. Very weird.
How to ask a friend if they are okay with Gays, Bisexuals, etc. And if there is a possible way to ask if they are one of them. But not feel totally awkward or weird about it?
I don't think there is a really good way to do that.
And that can make it difficult for someone who is themselves LGBT, and not sure how/whether to approach someone of the same sex, to see if there is the possibility of more than friendship. But:
1) When GLBT issues are in the news, that can lead to a discussion which might give you some clues.
2) Or something like: "People say that one of the very best floor shows in town is at a gay club near here. Have you seen it?" This can lead to wonderful good times, for gay AND straight.
3) Or, just allow the friendship to deepen without any questions, and eventually if the person is LGBT they will confide in you.
* * *
My friend Jim is near 80 now, and since infancy he has always known he was gay. Somehow over the years, he has developed an infallible sense of just knowing, that he calls 'gaydar.' Gaydar is the very most reliable sense I have ever seen - Jim is never wrong.
Not sure how good of a friend you truly are. Why does this matter so much to you?
It's better to mind your own business unless they offer to enlighten you. People are people, no matter what their sexual orientation.
On the other hand, and this may possibly help, I once met a guy who kept referring to somebody he called "my partner". After about three "my partners" I just asked, "What's your partner's name?" Once he'd told me it was George, or Fred, or whatever, the air cleared and he realised that I didn't give a hoot that he was gay, and he relaxed.