Like many others have said. You'll learn them through you're seat. But how I remembered which diagonal was right, I'd go like this:
When the horse strikes off into a trot look on the outside shoulder. If his outside shoulder is going forward when you're rising and moving backwards when you're sitting, you're on the right diangonal.
Rmember it like this; you go forward when the horse's shoulder goes forward. You go down when you're horse's shoulder goes down. I hoped that helped! Enjoy your future lessons!