The others have good ideas. You know in your heart he cheated, but you don't know for sure, so I guess I would tell them you really don't know. My son would then push me into a corner and say "But what do you THINK?" I would tell him the only one that knows for sure is his father and that I can't speak for what he thinks or does. If he pushed it I would probably even suggest he visit with his father about it. I would know he wouldn't but it puts the responsibility of it right back there. I also have turned the question on my kids as Abbey and ladyrb suggested. Good luck.