bofriend problems please help me.?

[petiteuncle563]

Me and my boyfriend have been together since 2012. He cheated on me in the past but we worked it out. Recently, two people have confronted me that he is cheating on me again. One said that they f*cked and the other said that they "hooked up" and of course I told him and he said no. But I don't know if I should believe it? I mean I do but then a part of me doesn't. I don't know what to do. What are some tips or ways to know that he might be cheating on me. Oh and I never met these girls so I thought it was weird. They said something to me on twittering through direct messaging. Someone tell me what I should do..