On a horse, you trust pretty well, pick up the correct lead and canter around the ring once or twice, closing your eyes when you can (obviously not a great idea if someone else is riding in the ring at the same time as you or your horse is spooky or whatever... use your discretion). As you canter, with your eyes closed, really try to feel the way your hips move, the way the horse's body moves. See if you can feel when his back outside foot hits the ground, then when his back inside and front outside hit and then when he places his front inside foot.
Try it in the other direction.
Now try picking up the counter canter and see how that feels different.
After a while of really trying to feel the motion of the canter, you'll be able to feel a distinct difference between the left lead and the right lead. You can test it, by going over a jump, and deciding which lead you think you're on, without looking down, and then check to see if you were right!