1. He's superprotective of his gadgets. "The main way that trysts are found out is through the discovery of incriminating e-mails, IM chats, cell phone texts or bills," says Belisa Vranich, PsyD, a clinical psychologist in New York City. So if he's being unfaithful, he may guard his gadgets or act really defensive when you innocently touch his phone or computer. It should be a giant red flag if he readily gave you passwords in the past, and now he's more evasive.

2. He steps up the grooming. This is so obvious, but it's a sign many women miss: "If your man starts grooming more without you requesting it, that could be an indication that he's getting intimate with someone else," says Vranich. You can actually thank modern mass media for this tipoff. Guys today are used to viewing manscaped dudes onscreen, so if he has another chick to impress with his appearance, he may emulate those ultra-trimmed guys. Another clue: He's spending more time at the gym.

3. He smells different. "When he comes home, if he doesn't smell the same as he did in the morning, and it isn't the scent of soap in the gym or at your home, it may be because he's showered at her place," offers Vranich. So pay attention, because in this case, that old saying "the nose knows" might very well be true.

4. Nothing fazes him anymore. "If he was short-tempered before, a combination of added intimacy and attention could be making him way more relaxed, even downright giddy," Vranich says. Adds Mira Kirshenbaum, author of "When Good People Have Affairs: Inside the Hearts and Minds of People in Two Relationships": " If your guy is suddenly going around all happy and whistling, then you need to find out why."

5. He becomes suspicious of you. "If he's normally a mellow type, all of a sudden he may want to know where you are all the time and with whom," says Vranich. "It's the result of him realizing that if he's cheating and it's not that hard, you might also be getting away with it." Also, beware of extremely detailed responses to even your most innocent "How was work today?" queries. He may be preparing epic answers because he's terrified of getting caught.

One caveat: If your romantic life hasn't fallen off, that's no guarantee that he's faithful. "It's a serious mistake to think that affairs are necessarily physical. He may just be unhappy in other parts of the relationship," says Kirshenbaum. In fact, an illicit relationship could even stoke his lust for you.