do you believe to know if person is a g_y, get him drunk?

I don't see how that will prove they are gay. It's possible that they may do something just because they are drunk, not because they are really gay. Unless you just mean that they will tell the truth. I guess if they were gay but hiding it that if drunk they may let the truth slip out. I don't think that is the way to go about it though. You either should be honest and direct and ask them or else leave them go till they are ready to tell you about it.