Thanks a lot this has helped me alot
I was taught to look at the inside shoulder and when that leg is back you should be posting up
When I first started learning about diagonals I would look at the front outside leg (the one on the rail). When that leg is forward you should be on the up part of the post. When it is back you should be down. Make it a habit to check right when you start posting, eventually you will be able to feel if you are on the correct or incorrect diagonal.
Hope this helps!