How do you know if a guy is secretly Gay??And not just bisexual as he claims to be...?

Thus guy likes me but he wont ask me out cuz hes afraid of what could happen,and also it takes a long time for him to trust somebody.Then I also thought hes just keeping a hold of me just incase he cant find anything "better" I could be overthinking about that one tho... But ik hes bi..and on tumbler he has gay flags and everything is colorful..(if that means anything)how do ik he likes boys more..?