You are out shopping and you see your dad walking hand in hand with an attractive young woman who is definitely not your mother. You watch them walk into a restaurant, find a booth in the back and begin kissing and cuddling.My answer:
You follow them into the restaurant and you father sees you as you approach them. He jumps up and begs you not to tell your mother what you’ve seen he says the affair is only a week old and he will end it right now and never see the woman again. Do you tell your mother what you know?
My immediate question would not be "What's Dad doing with a hot young woman?" it would be "What'd Dad doing all the way out here in Oregon without having told me?" My second question would be "Did I see that correctly?" then "Nah, couldn't be. Maybe he has a twin brother he never told me about."
If I went in and verified his identity (he has a distinctive cough caused by a kind of hernia in his throat that everyone in our family can recognize immediately even without seeing him). If the second paragraph were to happen (almost as unlikely as Dad having an affair to begin with), I'd give him the benefit of the doubt. But I'd want to know why he was in Oregon and not in Iowa. :-)