The days of suspecting cheating based on phone calls or hang ups in the middle of the night are long gone.  Most people who are cheating are not going to give that person their home phone number or even their office number.  They are going to have their partner in crime call them on their cell phone because everyone always has their cell on their person and they can turn off the phone if they are with their wife or girlfriend.  Or, they can sneak off until they have relative privacy to make the call.  Or, they can communicate via text messages and turn off the alerts until later.  In short, the cell phone is one of the more perfect cheating tools, but it's also the one that will most often get them caught.

The Cell Phone Bill Doesn't Often Tell The Tale Of Cheating: In the old days, we could look at the bill to see what calls our partner was charged for.  This is mostly long gone too as many people have unlimited calls or minutes and are just charged a flat rate.  However, if your partner has a pay as you go or pay by the minute plan, then often you can look at their bills or log on to their account and look at the "account summary" or "account history" and this will list the numbers that have been charged.  Obviously, you'll want to take note of those you don't recognize and plug them into one of the reverse look ups that will tell you the name, address, etc of who they are calling. These services are often pretty cheap.  You pay around $20 for unlimited look ups.

The Call Log: Another way to find out any weird numbers is to look on the call log or missed calls on their cell.  Often, they will have deleted the voice mails and texts, but check to make sure.  And, they will sometimes delete the call log / missed calls but there is software that will allow you to get these back. Check the pictures too.

The Prepaid Throwaways That Cheaters Love: Often times, people who cheat know that they have to cover their tracks.  Although they often aren't thinking with their head, they will take some caution and they know that their phone leaves clues. Often, they will buy the cheap throw away phones that you can get at any drug store or super center and will use that just for the person that they are cheating with. 

If their regular phone doesn't yield any clues, then you must suspect the throwaway.  They will often lock this in their glove compartment or hide it in their office or locker. But, sometimes it's on their person and you can either find it that way or search around for it.  When you find it, the process is the same.  Look at voice mails, IMs, texts, and the call log.  Then, run any weird numbers through reverse look up and you'll have your answer.