1) Are you having less sex with your spouse than normal? Diminished sexual activity at home maybe a sign that your spouse is having sex outside of the home.
2) Has your spouses' personality undergone a change? Something caused it and it may be the presence of another lover.
3) Has your spouse less money to spend? If that person has not suffered a financial loss, then the money may be going to pay for dates, dinners, dances, etc.
4) Has your spouse recently begun devoting an inordinate amount of time to grooming, to worrying about dieting, and to buying new clothes. Such concerns may indicate a preoccupation with appearance, in an effort to impress someone.
5) Is your spouse absent from home more than usual?
6) Does your spouse spend more time on the cell phone / internet or is your spouse more secretive about checking their email.
7) Unexpected behavioral changes tend to be a tell tale sign of something taking place outside of the home.