First do a self-examination. How you do this is by looking at the moles, freckles, blemishes, and other kinds of marks on your skin to see how they change over time and look around your body to see if there are new marks that form.
So what are you trying to look for? Just try to see if any marks feel or look unusual. This does not necessarily mean skin cancer right off the bat, but it could be warnings to be aware of that.
You should also remember ABCDEFG when you look at your moles. A is for asymmetrical because normal moles are relatively symmetrical. B is for border, meaning that if the border is blurred, notched, or irregular in any way. C is for color. Moles typically are one color throughout, but moles with inconsistent coloration are not normal. D is for diameter. Signs of moles that could turn into skin cancer are usually 6 mm in diameter (although they can be smaller sometimes). E is elevated, as in the mole is not flat. F means firm to touch. And lastly, G is growing, meaning the mole grows and changes over a short period of time (like a few days or weeks).
If you have any signs of this, contact your doctor immediately. If not, continue doing self-examinations at least once a month.