I like The Onion article where a guy talks about his "amazing blackdar" whereby he can tell if someone is black by just looking at them.

heh. :<) My youngest, who is adopted, and who, I suspect, would easily trigger this person's amazing skill, has "white" on her birth certificate, whether by virtue of the adoption or from the birth mother I never inquired.

That said, I've usually heard -dar as connected to whatever group you're looking for, not the general "gaydar".

I had heard some of these - that seems a more reasonable extension.