How can you tell the difference between subungual hematoma vs melanoma? had a dark area under toe nail for 3 months (not a vertical line though)

Not safe to guess.: Telling the difference between the two is not always as simple as following as bullet point list. Both of these conditions can vary in appearance from patient to patient. To reduce the chances of missing the diagnosis, several things are needed. A thorough examination, a precise history of the problem and most importantly, experience in recognizing what you are seeing. Have the examined and soon.

Melanoma: I would suggest you see a doctor if u are concerned. Melanoma is usually seen as a vertical pigment pattern . It can be seen on dermoscopy. Do not self diagnose as these are two contrasting diagnosis in terms of risk.