I was kind of confused when I read this but I think by leads, you are talking about diagonals while trotting. Leads are for when you're cantering and diagonals are for when you're trotting.
When you are posting to the trot and you want to be on the correct diagonal, you will "rise and fall with the leg on the wall." That's how I always remembered it. If your are traveling to the right, when the horse's left front leg comes forward you will rise. So when that leg hits the ground and the right front comes forward, you will sit. And then reverse it when heading to the left.
For leads while cantering, a correct canter stride starts with the outside hindleg. Say we are traveling to the right again. The first hoof to start the stride will be the left hind leg, followed by the right hind and left foreleg at the same time, finishing with the right foreleg. The right foreleg will typically reach out farther than the left foreleg and that is how most people tell which lead they are on. In order to be correct, especially on horses who are incorrect up front or behind, you will watch and feel for the hind end. :)
I hope that's not too confusing!