I have always said it is only the act that should be punished, which would require witnesses. I have actually said this over and over again. I also said above that therapy should be the preferred method of dealing with homosexuality itself. I am not advocating execution on suspicion, but for some reason any mention of capital punishment for homosexuality as the Torah requires automatically makes people jump to the conclusion that I want to start rounding up everyone with a lisp and start hacking.
Witnesses duh, the act is the key term. If two guys kiss, which I've been blessed enough to have never had to witness it except for Hollywood, it can't be for that, some people might just be extra weird or something, you have to catch them in the act. It can't just be a confession either, because of the bad things that could happen. If they say/confess it, that attracts suspicion.