I think my fiancé is cheating...

We have been going through rough times lately and I feel like our relationship is not working out... last night he got a notification on his phone but they didn’t pop up in the lock screen, I just heard it I told him “I was right” he said “about” “I’m like ur hiding something if your notifications don’t pop up” he didn’t say anything. Now, today he was going to post a story on messenger and I could see what he was doing with his fingers and it seemed as he deleted something I automatically stopped talking to him and was having an attitude.. after that He showed me some msgs saying he’s going to a bar tonight wit his homies. I told him I don’t want him hiding anything from me or lying to me. But I also don’t want to be that girl who goes through his phone Bc we dont share our passwords.. we’ve been together for 4 years and our communication is not the best... idk how to come about me finding out whether he is or not..