We’re torn here. We don’t want to recommend that you go through your partner’s phone. It signals a complete lack of trust. If they’re innocent, you’ll feel better. If they catch you, they’ll be upset. If they’re guilty, they’ll put you on the defensive for flipping through their phone.
So, as an absolute last resort — i.e., several other signs of cheating have been determined — you can take a casual glance at your partner’s phone if they leave it lying around. Look through the messages, emails and browser history. If you find proof, take pictures with your phone.