Well for a start, a number of thoughts and questions would run through my mind and knowing the kind of person i am, I'd not rush to act but wait patiently and watch from afar till she gets into the house. While watching from afar I'll be noting the details and would use it to ask her questions about what i just saw this is because i believe a lot in giving people the benefit of the doubt and a chance to explain themselves even if i know better.
I'd allow come into the house and settle in, then I'd throw the questions to her as if i didn't see anything just to ascertain if she'll come out clean and tell me the truth. If she tells the truth then i know there is no cause for alarm but would still ask her some other questions from the little details i picked up watching afar off but if she brush off the initial question and decides to tell a lie instead, i definitely know I'm been cheated on so i wouldn't waste my time to tell her i know the truth and practically draw the curtain on the marriage because one thing i detest most is a lying and cheating spouse and it can easily provoke to make rash decisions with unintented consequences.