Oh, Don, you could probably feel it, but why don't you just take a peek and see which shoulder you're rising on. Purists would suggest that you should not look down - changes your posture - but just go ahead and cheat this one time.
If you're rising when the horse's left shoulder is moving forward, you're on the left diagonal. If you rise when his right shoulder goes forward, you're on the right diagonal.
I suspect most horses are smoother when we post on one diagonal than the other so we're inclined to post to that diagonal, but if you make an effort to use each diagonal equally, the horse should eventually even out.
