1. Go up to a complete stranger with dreadlocks and ask them if they are Jamaican.
2. Regardless how they answer #1, ask them if they are Rastafarian.
3. Regardless how they answer #2, ask them if they smoke weed.
4. Regardless how they answer #3, ask them if you then can buy some weed off of them.
5. Regardless how they answer any of the other previous questions, finally ask them if you can touch their hair.
When the person with dreadlocks tries to get away from you (by this point, you should have thoroughly offended and insulted them) just dig your hands into their scalp and touch their hair anyway without permission. This works best if you are a white person and the person with dreadlocks is black.
There. Five easy questions guaranteed to piss off anyone with dreadlocks. I speak from personal experience. Have fun, minions! I think black eyes and split lips are in style right now. :)