Talk to his former significant others. If you know of anyone your man was previously involved with, contact him or her and ask about his behavior. Ask past significant others if they ever suspected him of cheating, what made them think so and how they found out for sure. If he has cheated on lovers in the past they might be able to clue you in to behaviors and telltale signs to look for. Remember to take this information with the proverbial grain of salt, however; past lovers who harbor resentment or still have feelings for him might try to mislead you just to stir up trouble.
Wash his laundry. Cheating men leave a lot of clues to their infidelity behind in the wash. The obvious signs of lipstick or makeup on the collar or another woman's phone number left in a trousers pocket is an obvious sign that he's on the down-low with another woman. Also, check for cigarette smoke if your man isn't a smoker as this could be a sign that he was at a bar or a club without you, meeting someone else. Another subtle sign is if there seems to be an unusually high volume of dirty laundry. This could indicate that your man has been changing his underwear and clothes more often than usual to avoid being found out.
Check his phone. When your man isn't around, look at the record of past received and dialed calls on his phone. Even if he is smart enough to not program anyone's name into the phone, an inordinate number of unrecognized calls could indicate suspicious behavior. If you're feeling brave, you can always dial one or two of them and see who's on the other end. If he uses a computer and sends a lot of instant messages, you can also check this feature on his computer and look for new or suspicious contacts.
Surprise him. If your man is on the down-low it is likely that he is depending on your routine to stay the same in order to avoid getting caught. If he knows what time you come home from the gym or when you leave for work he will work around these times for meeting with someone else. Come home early from work unannounced one day, show up at his place of employment just to surprise him with lunch, or drive around the block a few times when he thinks you're headed for the gym and then head home and see what he's up to. If he has nothing to hide, he shouldn't mind the surprise; if he is hiding something, an ambush is a great way to catch him in the act.
Observe his behavior. People give away hints in their body language when they are hiding something. No matter how good a liar your man is there are ways to tell if he's up to something just by watching how he acts. Take note of whether he gets jumpy when the phone rings, if he goes into another room to take a call, if he starts making excuses about why he can't go places with you or if he suddenly has a lot of plans for work or with his friends that are causing him to spend more time away from home. These could all be signals that your man is on the down-low and you need to investigate further.