My father died of skin cancer when I was young, this brings my chances of getting skin cancer up considerably. I have spent the rest of my life educating myself on how not to get it.
The person at most risk is the white, white person who gets a lobster red tans. The one with darker skin (more melanin) is more genetically equipped for the sun so they are less at risk - but everyone should be careful.
You know those brown marks on your body? Some people call them moles, other people call them beauty marks... Cindy Crawford has one above her lip, if that gives you any indication. Those are what you have to watch closely. Here is a rough guide for risky vs. not risky:
If the spot is not symmetrical, it is more likely to be dangerous. Perfectly round or oval is better
If the spot is three dimensional, as in you can move it around with your finger, it is more risky
The more hair that is growing out of the spot, the safer it is
If the spot has several different shades of color, as in brown, reddish and black all on one spot, it is dangerous
Rubbing irritates the spot, so if it is somewhere that is frequently rubbed, like the waistline of your pants or under a bra strap, it is more dangerous
If you think you see a mole that is suspicious, see your doctor. The removal of moles (for non cosmetic purposes) is often covered by insurance. I get mine done by a plastic surgeon who hardly leaves a scar. Trust me, the little scar is far better looking than hideous mark (see above description).
My father loved everything natural and outdoors. What ended up making him sick was long hikes in the sun with a back-pack rubbing against a spot. I would suggest if you know someone who frequently gets sun-burns, treat it as you would treat a friend who is smoking. At least inform them if they do not know all of the risks.