How do you know if you have skin cancer on your back and butt?

Biopsy. See a doctor so that if needed a biopsy may be taken to be examined by a pathologist for definitive diagnosis.
Somebody else to see. You will need to have somebody else to exam you off course. If there is anything suspicious -then see a dermatologist--get it examined & biopsied -and that is how you know you have a cancer or not.
Exam&biopsy. You should be seen by your doctor , if suspicious he will do a biopsy then you will know.