Someone's ability to sense if someone is gay or not when it's not obvious.

The ability/gift of being able to detect homosexuality in other people.

the ability to sense--without being told and without obvious visual clues--that someone is gay.

A sense that allows for someone to detect homosexuality in another

The gift of being able to sense the homosexual vibe.

Extended form : Gay Radar

GAYDAR. Short for Gay-Radar. The ability to tell when someone near you is homosexual, even if they have given no obvious indications of being so. This is an ability usually possesed by homosexuals and their fag-hags.
It's like Spiderman and his Spider sense. He can just "feel" when there is danger nearby. Gaydar allows you to "feel" when there is gayness nearby.

Similar to Spidermans Spidey-sense, except where Spiderman's spidey-sense detects imminent danger, a gaydar detects a homosexual nearby, either thru hearing the tone of one's voice or observing the style of their movement, which sets off the realization that this person may prefer members of the same sex. (Being a homosexual, deal with the definition, because most should defend this one!)