A girl I went to high school with was diagnosed a couple of years ago with invasive squamous cell carcinoma. It started in a place on her back. She had surgery and has not had a recurrence, thank goodness. Her husband spotted a dark spot and thought it looked suspicious. Thank goodness he was paying attention!
A good rule is to have someone look at the back of your body for you, since you can't always see it. With every phone having a camera, also take picture of the spot for comparison's sake.
Turns out, her dermatologist said the spot was almost certainly caused by too much sun. She was a sun worshiper in high school.