How do I know if he still has feelings for her?

how do I know if my husband has feelings for another woman? my children and I moved out of our previous home and left my husband, he's a narcissist and wasn't very pleasant. Less than a month later, hubby called me to tell me about a crush he'd developed on a female co-worker. every time we were together after that for whatever reason, he would talk about this woman, and be all excited about potentially having a new girlfriend (we had been together for 18 years), he eagerly discussed divorce and having his vasectomy reversed in order to potentially have more children. this all broke my heart completely at the time even though I'd been more than happy when our relationship ended. Fast forward 12 months, and we are now back together. he assures me nothing happened with this woman, but my instincts tell me different, and while I've already confronted him about my feelings that he did indeed 'do something' with this woman, he has steadfastly denied it. He has blocked her on facebook and didn't keep her phone number when he purchased a new phone. how do I know if he still has feelings for her? It's eating me up inside. Does it really matter that much? our relationship is better now than it ever was but every so often, I get this niggling feeling that there's something he's not telling me.