To start off with: I'm not a pro, I don't have a lifetime experience with many horses and I don't claim to know what's right for horses. I claim to know what looks and feels right for me - and that's what I do with our horses. In the end, that's the only reference I can go by.
Bear in mind that you can do something all your life and it can be wrong. My horse is very easy going and does not complain much. That doesn't mean I am riding him "properly".
How can you ever identify "proper" riding? (Is there a "proper" way to ride at all?) How would you know that those texts from the 18th century contain the truth about riding? How would you know I'm not making things up when I told you how to turn on the hindquarters? How would you know that you have finally found the right teacher?
Wouldn't you have to rely on your gut feeling at one point? As much as I love to read the facts and figures about horse training, I think even more important than any formal training is to re-train my instincts and how to "talk" to a horse.
Talking to a horse as I understand it, doesn't require any special training, but rather - like Romy said - an open, listening, unprejudiced mind like a child learning to talk. The outcome of that process will be a very personal kind of language that contains some general words that can be understood by other horses or humans, but most of it is a secret, intimate language for just me and my horse.
But I'm babbling about communication and you asked questions about riding. The problem about riding is that there is no "proper" (in the sense of horse-friendly or healthy) way of doing it in my opinion. In terms of horse training I regard the horses' natural movements as the "proper" movementss. Riding is not natural and therefore everything I do on horseback can at best be a compromise between my fun and the horses' health.
What I did to get an insight into how collection feels like, or the correct frame, is that I watched my horse and others in free movement. Preferably at play, or in flight, or even at rest. I take videos and watch them in slow motion or even frame by frame. There's my paragon of movement I strive to achieve. If I can even keep that frame while I'm on top of the horse, then maybe I can even find a really healthy riding style...
Of course, horses can move pretty sloppy in the pasture too, but there are hints of perfection in every move.
To get a feeling for what I can do to elicit a collected movement for example, I try to give my horse as much freedom as possible - preferably the horse is completely free, in the pasture, among his friends. I work on the ground so I can see the whole horse. When I found a body language cue that gets me collection from the ground, I can try to transfer that cue to the saddle and see how it feels. When I film myself, I can also check if what it feels like and what it looks like is congruent.
Of course that is a very rough procedure and especially when it comes to riding things get so complex, but still I think this is usually a better reference point for me than most of the teachings I hear from professional trainers. But then again, that is only a gut feeling...