how do I know if I cum or not?

I've been having sex with the guy that took my V card for over a year and I'm not sure If I've ever came before it always feels great but I don't think I've ever came before his confidence is completely gone with me because ever other girl he's been with he's been able to make them cum, but not me. I'm not sure if I can even make myself cum I'm so confused. Any advice?