I have a pretty good gaydar tbh. Some things to note would be, their looks (do they look very fashionable? tight clothes, perfect hair), their voice (does it sound feminine?) music (do they listen to pop gurls, like us?

) mannerisms, the way they look at guys, hanging out with a lot of females, ect ect ect
Obviously it is possible for there to be someone who fits in this category and is still is straight, but they aren't very common to see irl
Or u could just be blunt and straight up ask them