Idk I just look down at the shoulders and when the outside shoulder goes forward, I go up. It's really easy to tell on my horse though. I've gotten so that I don't even have to look, I can just feel it. Try looking at the shoulders while you're walking and ingraining it in your mind what it looks like when that outside shoulder goes forward. It helps to practice posting at the walk so you can get the hang of going up when you see that outside shoulder go forward, and it's easier to concentrate at the walk than the trot when you're trying to get the feel of catching it by looking.
Does this help at all?