Should I push to find out or leave it in the past

About a year ago me and my boyfriend broke up for 4 months. When we got back together I asked him if there was anything that happened with other women. He told me no but in my gut I know he isn’t being honest. We have doing very well I’m happy but sometimes I let my mind wander and think about the past wondering if he told the truth or lied. My question is should I let it go or should I dig deeper?