There’s a few ways of telling, but this one caught my attention.

Unexplained time away

* They keep making excuses to go out and when they do they are gone longer than normal, e.g. a trip to the store for some milk takes an hour.

* They suddenly have to go somewhere, e.g. the store, gym, gas station. That may be a sign that they are meeting him or her.

* They frequently call to tell you they will be home late, e.g. they are helping out a friend, their truck broke down.

* They develop new interests that do not involve the family.

* They disappear at unexplained or unusual times.

* They are strangely inaccessible or unreachable at times.

* They tell you they will be home late, or need to spend more time at work (a.k.a wasting the electricity usage on the ceiling light).

* They claim to be doing overtime at work, but their checks get no larger.

* They take long lunches or dinner meetings.

* If you ask if you can meet them outside their work, they prefer you do not. (A lot of affairs start at the office.)