Ask open-ended questions and encourage them to talk. Don't look for details of the affair that will only make this more painful. If they deny that they acted badly, stay calm and present your evidence. Most experts agree that you should confront your husband about his deceptions.
But you need to have a plan. Choose the time and place carefully so that you can discuss the matter at length without interruption. It's important to have a planned approach to what to say to your cheating husband or wife to get the clarifying answers you need. When considering what to say to your cheating husband or wife, the main thing you want to know is why the affair happened.