• 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."