@mirza— Great answer. Ha.
Yes, I think it exists. If you’re aware of what to look for, it can be really easy. It also is a skill that can be developed. I grew up with a lot of gay “uncles” (my mom’s circle of friends), so straight out of the gate, I had a pretty refined gaydar for guys. However, I knew no lesbians, so at first, my lesbian-dar sucked really badly. I’ve been out for 10 years now and my lezdar has much improved in that time. I would say I’m usually right, but I have the added bonus of being pretty intuitive. I often get things about people I couldn’t have known otherwise.
Straight or gay people can have it, I think perhaps gay people have a slight advantage in that they probably have met many kinds of gay people and are more aware of what to look for.
It’s definitely not foolproof. There are effeminate straight men, and butch straight women. Lipstick lesbians always fool me.