What to do if you have doubts about your partner’s fidelity in an online relationship?

I've never had that question before. I don't know what I would do if I doubted the fidelity of my partner with whom I communicate at https://www.sofiadate.com/. Maybe I would stop communicating if I had undeniable proof that he was cheating. But so far I have not had such situations and I hope not to have them in the future.