Why not make up some kind of lie and say that your teacher at school just got cheated on by his wife or something? Maybe not a teacher, but someone that she wouldnt be able to find out about. Then see her reaction and ask her if she would ever do that to your dad. Then again see her reaction, and if she doesnt tell you then you might as well confront her about it. It then kinda means that she is trying to keep it a secret.
