How do u knowif u got skin cancer?

skin cancer is one of the most curable and most deadly cancers. If caught and removed there is nothing else to do but if left alone it will spread to your blood , lymph nodes and everywhere else. Signs to look for are moles that are new or have changed in appearance/color , moles with irregular borders or that are not symmetrical. Also if they are black in color they are most likely a melanoma and need to be removed ASAP. If you are concerned you should see a doctor ASAP.

See a dermatologist.
You go to the doctor
By seeing a dermatologist for a diagnosis.

If you have any questionable spots - the rule of thumb is ABC

Asymmetrical (is it lopsided or like a splotch vs. a round mole)
Border (again, round or splotch?)
Color (are there different colors or shades of color in the same mole?)

Any thing that has changed recently or just concerns you is worth checking out. Better safe than sorry, ya know?
go to the doctor... get a referral... go to a specialist, aka dermatologist... and they will do a biopsy. when the biopsy comes back the results will tell the Dr. and the Dr. will have the nurse call you, and that is how you will know.
a mole that changed color or a sore that won't heal