1. Missing in action.
Suddenly your husband or wife is gone a lot more often. "Sorry, I've got to work late again, dear." There may be excuses, but they don't always have a ring of truth. The time away seems excessive, and unusual. Logistically, your spouse can't be unfaithful while s/he's with you at home. The more time away, the greater the opportunity.
2. "Mirror, mirror on the wall."
Your spouse becomes preoccupied with his/her appearance. I think this symptom is a very big indicator. Remember how you acted when you first met and dated your spouse? You spiffed up, got some new clothes, went to the gym, and got a tan. You'll see these behaviors in an unfaithful spouse.
"Remember how you acted when you first met? You'll see these behaviors in an unfaithful spouse."
3. Your sex life changes.
It might dwindle to nearly nothing, where all the excitement in the bedroom is gone, and you can't get your spouse interested. You're hearing "I'm too tired," or "I've got a headache" night after night. You feel rejected and abandoned. Or, your spouse starts wanting to do strange things in the bedroom that make you uncomfortable.
4. Emotional withdrawal.
Your spouse doesn't speak to you about personal feelings. The communication between the two of you is very shallow--just business talk and stuff about the kids. Not only that, but your spouse seems aloof, detached, and even irritable. He might pick fights for no apparent reason. Or she doesn't care about family vacations, fixing up the house, or other domestic responsibilities.
5. Invasion of the "friendship" snatcher.
Your spouse spends time alone with someone of the opposite sex, who is referred to as a "good friend." This "good friend" seems inappropriate to you, and you've got a funny vibe about the person. But your spouse is extremely defensive of the friendship, even insulting you for suggesting something isn't right. While their friendship seems strong, you and your spouse are less friendly.
6. Telephone signals.
You're getting occasional hang-up calls where the caller ID is blocked. You haven't seen your spouse's cell phone bills, because they're sent to the office or quickly snatched up. If you do see the bill, there is one number (or several) that are repeated numerous times at odd hours. According to private investigators, cell phone bills are the number one way to catch a cheating spouse.
You've caught your spouse in a lie, or several, about his whereabouts or activities. Or, something he's told you doesn't make sense, but you can't quite pinpoint why. I remember writing some notes to myself about my spouse on a pad of paper: "What he's saying isn't adding up," I wrote. "I don't think he's being honest with me." He wasn't.
8. Money talks.
There is financial tension in the home. He or she is monopolizing the bank accounts and keeping you in the dark. Or, your spouse is antsy about where the money is going and wants more cash for personal use. You may notice that money has disappeared without a reasonable explanation. There may be strange bills, or missing bills. Your spouse seems unusually possessive with money.
"There may be strange bills, or missing bills."
9. Road signs.
There is unexplained mileage on the car, it's cleaner than usual, and your gas expenses have gone up.
10. Flickering flame.
Your spouse makes cryptic statements about being "unsure" of his feelings, eventually leading to the announcement that he (or she) "doesn't love you" anymore, and plans to leave. Although this is not a certain indicator of an affair, in most cases it's rare for a spouse to "jump ship" without another boat waiting down stream. In every case I've known personally, when the spouse announces that his love has flickered out, there's at least of spark of interest in someone else.