Gay/Bi how to tell without seeing?

Not sure if this goes here so if not redirect me

I sometimes jack off in my dinner break at work, get some porn on my phone and find an empty bathroom stall in a (usually empty) bathroom. so the other day im doing what i normally do and someone walks in and takes the stall next to me, no problem i'll just keep it quieter.

Obviously after a bit my breathing get heavier and more rhythmic but i also notice that whoever is next door to me sounds the same heavy breathing and almost in time with mine.

Now im pretty sure im straight but Ive always had fantasies about guys and this was getting me off I loved the face that there was someone there jacking with me anomalously but was he?

Is there a way gay/bi guys know this? I cant really knock on the door and ask him to let me in and along the same lines i would love to do it again but how can I be subtle about this?