To develop the canter, you need to make sure you can ride forwards and back in the canter by making tempo changes. This tests the reaction to the aids: when you put the leg on the horse should instantly go forward, and when you ask, using the seat, for the horse to come back, the horse should slow the rhythm and begin to sit. 

Not only does this test how well the horse is responding to the aids, it is also developing the horse’s canter and the ability to half halt, which is very important when teaching the flying change.