To catch a cheating spouse, monitor incoming calls, receipts and credit card usage. Check if your spouse is making unusual and frequent calls and being secretive about it. You can also turn to various kinds of surveillance equipment. Do not let him know that you are suspecting anything until you are sure.
A sudden change in your spouse's behavior may lead you to believe that he might be having an affair. Before you confront him, make sure that this really is the case.
First, keep track of your spouse's activities. Check his phone to see if there are unusual calls, especially frequent ones. Check his receipts to see if he is making suspicious purchases. Monitor his credit card usage to learn about where he is going and the items on which he is spending his money, such as jewelry, hotels and romantic restaurants.
Consider purchasing software to monitor online activity, cell phone activity monitoring applications, GPS tracking devices that constantly display the location of your spouse's vehicle, easy-to-hide cameras, and voice-activated recording devices, all of which are readily available through online sources. The ways in which these programs and devices can be legally used varies by state.
Alternatively, hire a private investigator.