How do you know if a small lump is skin cancer?

There are a # of: types of skin cancer. Biopsy can provide definitive diagnosis.

You really need to : see dermatologist for diagnosis and possible definitive biopsy. You are looking for "lumps" that bleed, are growing in size, hurt, etc

Exam needed: An examination is the first step in the evaluation of lumps. Your doctor will take a thorough history and do a physical exam. The information gleaned will help determine what other tests might be needed to make a diagnosis.

Have it looked at..: by a dermatologist or family practice doctor who look at a lot of skin lesions and can take a biopsy if necessary to confirm the presence of a cancer if that is the suspicion.

Biopsy: Lumps under the skin can be benign (not cancerous) or due to cancer. Most lumps are not cancer. Noncancerous causes include scar tissue, infection, inflammation, cysts,etc. If your doctor thinks it could be due to cancer, a biopsy can be done: taking all or part of the lump out and sending it for examination by a pathologist. This can determine if it is cancer or not and what treatment is best