How do you trust your boyfriend?

how do you trust your boyfriend again? when you know he cheated well we were separated at the time> But i know he other relation and he denies it . But its important to me because safety reasons and it shows me he ihnost. I love him but for some reason im having a hard time trusting him. He has always had alot of female friends . But i just don't know what to think at this moment. We wonna marry but i wonna do a lie detector test . I need advice