The signs of a cheating spouse include direct evidence, such as lipstick stains on the collars, as well as more circumstantial ones, such as long-standing changes in pattern of behavior. Many behavioral changes can have innocent explanations, however, so it is best to avoid overreacting.
In addition to the classic "lipstick on the collar" warning signs, other strong pieces of direct evidence of an affair include unfamiliar hairs in the clothes or car, a change in birth control regimen (such as a husband who buys condoms in a marriage that does not usually use them), and unexplainable bills or purchases, especially for items such as a new cell phone or nights at a hotel.
More circumstantial signs include a spouse who suddenly stays at work later than usual, or goes out after work more than was customary. Changes in dress or grooming habits can also be a sign of an affair, particularly when the spouse is dressing better than he or she did before. Close guarding of phones and computers, both with physical proximity and new passwords, is another potential warning sign.
It is best to be cautious with these more circumstantial signs, however, as some of them may be explainable by such events as a new boss, the chance at a new promotion, or new cyber security measures undertaken by an employer. A spouse who is secretive or defensive about such patterns is more likely to be cheating than one who is open and honest.