Pronation is when you turn your feet so that the face out, supination (or underpronation) is when you turn your feet so the soles face in.
Look at the soles of your shoes. If they are obviously more worn on the outside of the heel you're a supinator. If they're obviously more worn on the inside of the heel, you're a pronator. If the heel is worn fairly centrally, you're neutral.
I went to a local store called Fleet Fleet for an evaluation, but I wasn't satisfied with the service. They wanted to sell me more stuff than evaluate my stride so I could get good shoes that would reduce my shin splints and knee pain from running. If you have a place that does do the evaluation then that is awesome, but if you do not there is a easy way to see what your style is.
Wet the bottom of your foot by dipping it in water. Place a piece of paper or a brown paper bag on the floor in front of you. Step forward in a normal stride, rolling your wet foot on the paper. Follow through with your stride by stepping forward with your other foot to get a better personalized picture of your pronation.
Examine the wet mark on the bag or paper:
Full foot indicates overpronation
Narrow arch -sometimes with a separation between your heel and the ball of your foot -- indicates supination.
Normal pronation image will show about half of your arch
Here is mine:
I used construction paper. As you can see my pronation style is supination (or underpronation).
Check it out...it is pretty neat. You don't have to go anywhere to find out now. :)