Learn something new every day. (And just how do you prove you're gay?)
Assuming you are a man, I would say by having sex with another man.
(And just how do you prove you’re gay?)
Right now, blood banks rely on the honor system. They inquire at the time of donation if the prospective donor has had homosexual sex since the 1970’s, among other queries.
Anyone who says “yes” is not allowed to donate.
The policy makes good sense. Besides aids, active homosexuals can be a vector for hepatitis as well as other blood borne diseases especially including those not yet identified or isolated.