A man who's having an affair will almost always leave signs that you can pick up. He may spend more time working on his appearance and on his computer, spend money he can't account for and seem distant and distracted when he's with you. Some cheating spouses use secret cell phones to communicate with their other partners. If you can find a secret phone, you'll know your suspicions are valid. Before you go down this path, though, do a gut check and make sure you're not jumping to conclusions. Snooping through your husband's things is a violation of his privacy, so you should be fairly sure he's cheating before you do it.
Pay attention to your husband's habits. When he walks in the door, where is the first place he goes? Check in that room, in every hiding place you can think of, for the cell phone. If he has one, he is likely to turn it off and hide it when home. If your husband feels you are suspicious, he is more likely to try harder to hide the phone, so avoid alerting him to your suspicions until you have proof.
Find an excuse to get in his car. Look in the glove box, under seats and in the trunk. Check every part of the car you can. Again, if your husband suspects you are on to him, he will take extra care to cover his tracks. If he is unaware you are concerned, he may place the phone in a very easy-to-discover location.
Check in all other areas of the home if you are unable to locate the phone. This could be the garage before he even comes into the house. Check his briefcase or any work bag he carries.
Make it a point to check the mail before your husband. Chances are your husband has the mail forwarded to another address or uses electronic billing, unless he is certain he always checks the mail. If he does, check the mail in the mailbox but leave it there for him to pick up so he doesn't know you are looking.
Get on your family computer and check the web browser history. In your browser window, click the down arrow to see all the recent pages viewed. Look for cell phone companies. Check his email if possible to find emailed bills or emails from suspicious people.
Prepare to give up the fight if you are unable to locate the cell phone. It doesn't mean he is not having an affair, but he may not have a secret cell phone. If you are still convinced there is an affair consider finding other evidence of an affair such as missing money from your bank account, him not coming home from work, getting a GPS tracker for his car, or adding key logger software to his computer.