A friend and I discovered yesterday we learned diagonals the opposite ways. I learned to check the inside leg (when the inside leg is forward, you're sitting down). She was taught to check the outside leg. Of course, now we feel them, but we started wondering- is it a discipline thing? My mom taught hunters, so I learned from her in the 70's. My friend started out eventing (she's about 10 years younger than me).
I was taught to follow the outside front leg... unless there's a show photographer... then you switch so you won't be airborn for your photo (Saddle Seat photographers almost always catch the outside leg at the height of the stride. It makes the front end look bigger. So if you switch, you will be nicely seated for you portrait). In Saddle Seat Equitation, the outside diagonal is the correct one.
ETA: with this horse I always post the outside front diagonal. He is MUCH easier to bend around a turn if I sit with the inside hind.
Well, there went the idea that it was a discipline thing! Interesting about the saddle seat and the picture opp.
Good question about feel... do you think all riders can or have the ability to? I see so many people look down, and I assume it's something they habitually do, and they just haven't tried to feel their diagonals (or, nobody suggested it to them!).
I used to teach up-downers and frankly I have taught it a lot of different ways. Outside leg, inside leg, side to side/feeling it, at the walk, from an established trot, pick one and fix it if it's wrong, pick it up right away...whatever seems to click for a particular student. Sometimes with kids especially you have to experiment a little, as different people learn differently. As long as you find a way that helps someone feel it and then learn to be consistent, it doesn't matter.
I teach and ride saddle seat. On many saddle seat horses you can eaily see their knee when trotting, so being up when you can see the horse's knee is very easy!
Recently had to reteach an adult rider who had been taught to watch the inside shoulder by a hunt seat instructor, and perhaps it was just this rider, but she didn't really know her diagonals. Teaching her to look at the outside shoulder has greatly improved her ability to tell her own diagonal.
Feeling your diagonal is ideal, but I would rather a rider take 5-10 seconds to glance with their eyes and be sure of themself then waste a whole straightaway trying to feel.