My gaydar is really good.
I know everyone says that, and there's little reason to believe that a generally socially inept computer programmer can have an above average gaydar, but you're just going to have to believe me that for some strange reason, this is an area where I demonstrate competence. I don't know whether it's the voice, the mannerisms, where their eyes track, or what's left unsaid, but I have a decent level of accuracy in determining gayness.
My lezdar, however, is abysmal.
I don't know why this is. I live in an environment with an above-average density of lesbian women, but I'm still just terrible at guessing who is heterosexual and who isn't. I can't tell the I-have-short-hair-because-I'm-a-lesbian woman from the I-have-short-hair-because-I'm-a-no-nonsense woman, and don't even start me on lipstick lesbians. They're impenetrable in all ways to me.
Here's my best theory on the difference.
With gay men, there's the merest hint of flirtation, and at least an instant of a moment where the guy is deciding whether I'm someone he'd like to sleep with. Obviously once I start talking, or when they get a good look at me, they quickly realize that I'm not the droid they're looking for, but that previous millisecond of wondering is often enough to out them.
Women, however, straight or not, do not require that millisecond.