I've been having this problem lately: my qh canter is so short strided when she picks it up from walk that it's nearly impossible for me to tell if she's on correct lead. Even when I look down on her shoulders - still... I mean if we are turning I can tell, but on straight line it's really hard (unless I push her into bigger canter). I used help on ground today (my mom), who was keeping eye on her, but such help is not alway around unfortunately.
BTW, I have no problem to tell when I ride my paint (she has big forward canter).