Is your spouse carrying on an extracurricular fling, or is it something you're just imagining? If your hunch is right and your mate really is involved with someone else, count on your lover to leave some clues.

"There are always signs," says Marcella Bakur Weiner, Ph.D., a relationship expert with a private psychology practice in Manhattan and Brooklyn, N.Y. "Nobody accomplishes a perfect cover-up," she adds. You can catch the Casanova ... if you know the tip-offs to adultery.

Relationship Reality Check

If your partner is two-timing, expect some of the clues to be emotional ones. According to Weiner, co-author with private investigator Raymond Green of 180 Telltale Signs Mates Are Cheating and How to Catch Them (New Horizon Press, 2002), your suspicions should be spiked if your mate:

• Pays you more attention than ever, or buys you presents ("guilt gifts") — especially early on in the infidelity.
• Picks more fights than usual, and is consistently moody, inconsiderate and critical, but doesn't want to talk about it.
• Isn't saying "I love you" anymore and no longer shows affection or has much interest in sex. (Or, after all these years, your partner brings new tricks to the bedroom for you to try.)
• Chooses to spend more time than ever with friends or at work — or claims to be doing so — rather than with family, and responds with delay to your cell phone messages.
• Seems to feel guilty when you do something nice (your mate is betraying you, and here you are treating him or her like gold).
• Responds defensively to your normal, everyday inquires, e.g., "Why are you always checking up on me?"
• Accuses you of cheating from out of the blue.

Make No Mistake

Your gut often tells you there's something gone awry. "Women have a kind of intuition to identify behavior that's even a bit unusual," Weiner says — but you can't be sure without more indisputable proof. To collect the evidence, Weiner says to be sneaky if need be, suggesting you check pockets, listen to phone messages, and even take a close look at credit card statements.

Experts say these additional hints can help catch a drifter:

• Some bills aren't coming to your house anymore — your spouse has likely diverted them to a work e-mail or P.O. box so credit card and phone charges won't give the affair away.
• Your significant other is going through cash much more quickly than usual, or withdrawing money from a strange location (check any ATM receipts you can find).
• Excessive mileage is appearing on the car's odometer — your mate says s/he's just going to work and back home, but that's not a 42-mile round trip!
• Your spouse has new tastes in music — used to hate country, but now can't stop two-stepping? — or is working out and placing more emphasis on clothes and general appearance.

• Your spouse has taken to using new catch phrases and has opinions that are unusual for him or her.
• Your spouse's sleep patterns have changed, i.e., your mate's experiencing nightmares, sleep-talking (speaking another's name?), or exhibits unusual restlessness and exhaustion.
• You're noticing other, classic clues — lipstick on the collar, hang-ups or hushed conversations on the phone, or unfamiliar scents on clothing, for example.

If you're seeing these types of signs, keep a journal to remember all the clues, and to get a private detective up to speed if it ever comes to that. Write down the time of your entries so they'll point to any patterns, and keep them as impartial as possible, recommends Paul Dank, owner of the Michigan-based private detective agency Advanced Surveillance Group. P.I. Dank's sample journal entry: "He came home. He didn't have his tie. He said he left it at work."

Time to Pay a Professional?

When you finally confront a suspect spouse, he or she might deny wrongdoing — after all, the act isn't (yet) caught on tape. If you want proof beyond a doubt of your partner's philandering ways, consider hiring a P.I. to dig up the undeniable dirt.

"When people pick up the phone to call me, they can no longer accept the story of 'You're crazy' and 'You're making this up,'" says Dank, whose investigators conduct surveillance for hundreds of suspicious spouses each month. (Men haven't cornered the market on cheating, either — about 40 percent of Dank's clients are males who think their wives have gone astray.)

Private eyes, on average, charge about $1,500 for a basic two-day surveillance. They know the tools of the trade, such as computer spyware that can bust someone by their adulterous online communications. But is it really worth hiring a pro to prove your point?

Definitely do it if you're getting in deep and could be in danger. Following an alleged cheater is one thing that is always best left to the specialists, recommends Dank. He warns those amateur spies who are tempted to take a full-on surveillance into their own hands, "Crimes of passion are a very real thing."