I believe you already pretty much asked this question, and it was answered. The only way you would ever know you had a melanoma for certain is to get a biopsy. To do that, you'd have to see a dermatologist or other specialist to have a look at it. Not all melanomas kill people. You can either sit and worry about what seems to have been a benign growth on what seems to be a young person, or you can go see a doctor and see if you actually have a problem, which it doesn't seem you do. As was said before, melanomas generally are characterized by sudden irregular changes in something that was there for awhile, but again, you can't diagnose one by obsessing over it but you can drive yourself nuts. If you're old enough to develop one and you think you have one, go see a doctor.