how can you tell if a straight guy secretly likes guys? ?

Rule of thumb: they don’t.

Being gay is seen as pretty much a celebrity status these days. Everybody praises you, Ellen brings you on her show, everybody says you’re amazing and brave, and if anybody bashes you for any reason – even a legitimate character flaw or past actions that does not have to do with your sexuality – they get demonized by their peers because you’re seen as “a protected class” since you’re gay.

So believe me, if a guy even sort of liked guys, he would jump at the chance to come out as being gay. He would get all sorts of benefits. 

So if a guy says he is straight in 2020… Believe him

Being straight or gay isn’t like a light switch, it’s more analogous to a dimmer switch. Someone can be “straight” and push the boundaries of heterosexuality once in a while and vice versa.  If someone wants to keep their sexuality private why not let them?  What do you have to gain by outing someone?