How do you know which wire goes where on a three prong dryer cord?

most dryer cords are flat and the connection on the dryer is split or the ground wire connection will be at a different height. The cord probably has black, red, and white wire. Black and red are hot and white is ground. It does not matter black and red or red and black, they are both hot. If the dryer is not marked it will be layed out so that the ground will be the center of the connection.