How can I know what type of skin cancer I have before the biopsy? (photos)

4 years ago I found some blistered looking bumps on my left front of my shoulder just below the clavicle, itchy and very painful to the touch, my primary told me it was shingles and eventually would crust over and go away, here it is Four years later and still there, not much bigger if any than it was , a new doctor tells me it looks like cancer, I go to a dermatologist and he also says it looks like skin cancer but gave me very little information before scheduling a removal surgery