Would I be able to tell what dating sites someone is on?

No, they are totally confidential.

If you are looking for someone in particular that you already know, just tell them you are interested. Just ask them out, it's ok for the girl to do the asking. men quite like to be asked too.

If it is someone you are in a relationship with, then whether they are or not it's not going to work out. If you suspect them and they aren't doing anything then your suspicious, jealous nature will sour things. If they are then the relationship is already over.