How do you know if you are on the right diagonal in trot?

Answer 1

From April Reeves, Horseman's U.com: Diagonals were created to enhance and improve the movement of the horse as you post. Diagonals allow the outside front leg (in a bend) to extend farther (it has more distance to travel) AND to allow weight OFF the inside leg during the rise of that leg, since it carries the majority of the weight when circling or bending. “Rise and fall with the front leg on the wall” is often used to describe ‘when’ to rise the post, but when no wall is present, you rise with the outside front leg to the bend you are riding. If you are riding a straight line, you can ride either diagonal, especially on the trail. To keep you and your horse from tiring quickly or getting sore, change your diagonal often.

Answer 2

you should be rising with the outside shoulder of the horse if you are in an arena or a closed in area.