When looking to prove an affair there are many ways to go about it. When you think of your spouse, you will probably know your best course of action by thinking of their likes, dislikes, and general patterns of behavior. The biggest clue of where to begin is to consider all the places where your significant other has changed behavior.
Obtain cell phone records. One of the easiest places to approve an affair is with the phone. In today's world, especially with cell phones, while people might clear their text messages, the use of such messages are still logged. Cell phone bills now also list incoming phone numbers as well. By tracking down these numbers, who they belong to, and the usage to them, you can often times track down the person they are having the affair with.
Get work phone records. For those more discerning adults, who think to use their work phone or work cell phone, you can still usually get your hands on the phone bill. Sometimes it requires a little finagling, but even calling the office stating you are trying to track down a number for a "surprise" of some sort will oftentimes result in them handing over the bill as associated with your spouse's phone.
Track down computer and email passwords. Computer tracking is another way to figure out the fidelity of a spouse. If you know passwords it becomes fairly easy to see if there are any unusual emails. If you have this access, consider adding a bcc option to the outgoing emails from your spouse's account. Then any email they send out can be copied to your email without them knowing about it. This oftentimes allows for a great paper trail when it comes to a cheating husband or wife.
Computers can also be checked for cookies to unusual chat sites or flagging their chat account. With some help, a computer specialist can usually call up old conversations from computers if you are suspecting unusual chatting activity.
Gather financial records. When going over credit cards and bank accounts you can look for the obvious signs such as hotel charges or expensive dinners. Also consider looking for purchases such as jewelry, shoes, lingerie or flowers. These types of purchases can also often lead you to the address of the cheating party.