How do you know if someone is checking you out or interested, especially at the gym?

First of all, I'm autistic, AuDHD. I look at some people multiple times to analyze them when I'm not attracted to them at all, just curious so I guess potentially if they caught me they might think I find them hot.

If they look at me, I am mostly the only alternative looking person in my whole gym all the time, tattoos, piercings, my hair, so that's already a reason to look, and I also look if someone else like that is there.

I think I did spot a guy checking me out one time for sure but also another guy who looks at me every time but I don't know if hes into me or just curious of my looks.

Another thing, if I look at someone and check out their muscles, doesn't mean I'd actually hook up with them. Though when I look at my gym crush, I'd definitely give him head if he was interested.