I always explain gaydar to people this way:
When I see or meet someone new, I'll often get a sense about them, part empathy, part bottom of the barrel impressions from my "conventional" senses. These flashes often give me an intuition about a person. This sense or skill or whatever has been very useful in my life.
Whenever that sense tells me, "It takes one to know one", that's when I define the sense as my gaydar going off.
Perhaps not surprisingly, most people have a similar skill. Most call it simple intuition, but a lot of people ignore it, because intuition often seems, on the surface, baseless.
Since gaydar is, to me, based on intuition, and my gaydar is usually very reliable, I'm forced to wonder if people's intuitive sense is as strong as mine, and what I did to place such trust in my own intuition.