How do I know if he's cheating on me ?

So it's been a year that my boyfriend and me have been going out and I have been finding things. For example yesterday I found out that he added his ex-girlfriend on Facebook like a month ago and I berely knew about it. But today I found out he had tinder and there were messages. He told me that he didn't created but his bestfriend did on New year's so I forgave him and erased the account. My friend today told me that one of her friends found his tinder and that account was made when me and him berely had 5 months together and my friend send me the proof it looked like the account that we deleted but the account we deleted didn't have a bio and the one my friend showed me did. He came from work today and got a message that said that that person was from New York and I ask where did she or he got the number from they said from a site but I asked what site and they didn't respond and when I messaging this person my boyfriend got all mad and serious. Like if I was the one doing something wrong. I just want to know if there's a possibility that he's cheating on me.