They often show this in TV forensic crime dramas. It involves the use of wooden dowels a little smaller than the bullets used. And it is pretty conclusive. They can often tell if a person had their arms up when shot, or not.
Other factors are considered as well, such as powder burns on clothing and skin, how far the bullet traveled through the body, organs it hit enroute. You get two or three matching variables and it is pretty conclusive.
If Martin was straddling a prone Zimmerman and Zimmerman fired up through him, it would be very hard to prove a different shooting scenario.
My post stands.