My gaydar is honestly really good, though I realize that "gaydar" in itself is sort of based on stereotypes
Sometimes its facial structure and/or expressions, sometimes it's the way they walk (there have been studies on this), sometimes its their voice (but this is a little less reliable), and sometimes it's the fact that they're blatantly hitting on you or other guys
No "gaydar" is ever completely accurate though, and sexuality itself is very fluid .