A common symptom of a midlife crisis is restlessness and a desire to change life circumstances. If your spouse appears distant or shows a lack of interest in old hobbies and develops new hobbies, this may be a warning sign. If your spouse wears a new cologne or perfume and dresses better than usual, he or she may be dressing up for someone else. A cheating middle-aged spouse may ask about your schedule frequently, so that he or she can plan for romantic liaisons without you being the wiser. A man or woman who loses weight or stops wearing a wedding ring may be trying to impress or deceive someone else.

A lack of sexual desire in older men is normal, and may be a symptom of erectile dysfunction or low testosterone levels; however, it may also mean he is having an extramarital affair. If a spouse exhibits new talents in bed, he or she may be learning those talents from someone else. Requesting new acts in the bedroom or not showing normal affections such as kissing, cuddling or hugging may also be signs of infidelity.

A middle-aged man or woman who is making unexplained purchases on a credit card, or is making ATM withdrawals to hide purchases, may be spending that money on someone other than you. If your spouse makes long regular phone calls to a number with which you are not familiar, it may be a sign he or she is talking to another lover. Being secretive with a cell phone or purchasing an additional cell phone that he or she hides from you is usually a sign of infidelity.

Most men suffering from a middle-age crisis become bored with their current job, while women feel unfulfilled with their current lifestyle. A man or woman who works more hours or claims to be working late may be meeting someone else. Your spouse may tell you to stop calling him or her at work, or take additional business trips which he or she claims are due to business. If your husband or wife has been a steady worker at the same job for many years, look out for any changes in routine, as this may be a sign of infidelity.