My friend has informed me about my bf's promiscuious/unfaithful past & my bf is denying it, who do i trust?

My good friend, who I've lost touch with over the years but brought back together again through a coincidence, has been informing that my boyfriend hasn't been completely faithful during the course of our 3-year long 'on-again-off-again' relationship. I've never broken up with my boyfriend in regards to the issue of trust and I never doubted his loyalty until now. My boyfriend and my friend used to go to the same school and that is how I ran into my friend after several years of losing touch. She mentioned that he has a promiscuous nature and that he has tried to pursue her and her friend on several occasions. Of course, my boyfriend is denying all this. I don't really know who to believe.