To check or not to check?

My husband was going around with another woman for a long time behind my back (since early last year) and carried on lying to me and seeing her even after I caught him. His defence was always that they were just friends and business contacts and he wasn't doing anything wrong, so he didn't have to tell me anything. Well we had a big blow-up about her and all his lies back in June, and finally he agreed to keep things professional and impersonal with her. If I'm honest, I have my doubts how impersonal he is, because he got her a position in his office, and she keeps trying to get attention from him. He does tell me about her antics when I ask him, (like her asking him out for dinner, like old times) and he says he turns her down. It's true that he spends much more time with me now, and he's giving me all the right info that he doesn't give her the time of day. In fact the last two months, our relnship has improved very much. Yet somehow the last couple of days, I've been having doubts in my mind, and keep dreaming that he's carrying on with her. Last night he told me he's taking 3 colleagues out to dinner as a farewell treat for one of them who's leaving. There's no reason to suspect he's lying, but I keep wondering if he's actually meeting her again. Am considering following him tonight. If I can see him with the 3 colleagues as he said, I'll feel better, but it would disloyal to him. If I see him with her I don't know what I'll do. What should I do?