How? I know 'bad' dressage is unengaged hindquarters, on the forehand, forced head position, falling into the corners, etc etc, but how in the world can you tell? Especially from pictures, but even in videos it's kinda hard for me to identify.
It is actually difficult to see a lot in photos, especially as far as making firm statements about how enegetic the gaits are, how fluid, how supple, how rhythmic, all of which need to be seen in motion. At certain parts of the stride, for example, the canter always looks awkward. And you can't be 100% sure when you see a picture, if it was taken at a bad moment or if the horse/rider always look pretty much like that.
Some things you can see. Really glaring errors in rider position, very basic things such as cantering on the wrong lead - a good number of other things.
Too, if the rider is a beginner(and a person can even be a 'beginner' at Grand Prix, or at Grand Prix internationally or what have you, or they may be able to ride one horse well and not another), you might see certain things wrong, but they are things a beginner could never be expected to do so well.
The tests really help to guide what to look for at a given level. For example at training level, the directives say 'thrusting power' is important. So if a training level horse is shuffling along slowly without energy, that would be a directive for that level, and that's not being fulfilled. But again, there's something you can't always see from a photo.
In general, I'd really caution you to not spend toooooo much time on photos. Videos are better, and watching live is better, and watching live with an experienced person teaching you what's going on, is the best.
So here are some more specific questions:
How can you identify a forced head position?
If I saw a lot of photos of that horse/rider, and in all of them, the reins were hanging down and the horse's head was tucked in with slack in the rein, I'd be concerned the horse was 'set' in that position. Again, easier to see 'live', you can see the muscles in the neck rippling and moving and the head position changing slightly when it's not forced. There, it's easier to see if that slack in the rein is a momentary thing or a continuous problem with the contact.
Too, a beginner, due to having a weak seat and not being sure what to do, could have the horse in a 'forced looking' position, but he's just too inexperienced to fix everything at once. If he can steer and stay safe and follow the class, what can you say? Things take time. Hopefully he is on an appropriate horse that just 'keeps on truckin''.
And to some degree, everyone is a beginner at something in dressage. Even a top rider is always learning and improving.
How can you see if a horse is round/collected?
The collected horse has a different shape of stride - higher and shorter stride, his hind legs reach less far forward (but he's had so much training before this, in 'thrusting power', that when he's collected, his hind feet often will STILL land further forward than other horses without his training).
If you do not see a difference in the 'shape' of the stride, the horse is not collected. You should also see the joints of the leg bending and flexing more, and though the rider shouldn't pull the horse's head up with the reins, if the horse is collected, he will have his head and neck up higher than he does when not collected.
The horse should look light and agile in front, and he should look as if he's in a nice 'package', maintaining a contact with the reins, and despite being collected, the gaits should look natural, flowing, energetic, active and there should be no pauses, hovering or jerking motion.
The amount of collection is the least at 2nd level, and the most at Grand Prix. It might be hard to see the collection at 2nd level.
What should engaged hindquarters look like?
Engaged means that all the joints of the hind quarters and hind legs, have smaller angles, just as if you squashed a spring down. There are two kinds of 'engaged'. One, where the hind legs swing far forward and the hind feet land well forward. The other, is in collection, where the stride goes just a little more 'up' than 'reaching forward'.
How do you tell if the poll is the highest point? (Sometimes a horse has a big crest so it looks like it's not the highest)
Instead of looking at the top line of the neck, or the line of the mane or braids, look at the biggest muscle of the upper side of the neck which makes a definite line below the line of the mane, and see if its line continues UP in a continuous upward curve. Some horses have a 'long poll' and are deceptive, even when looking at the muscle.
(Oh, also, how do western-type horses get collected with their necks so far down? I've never understood the 'U' example.)
It's a different riding sport, and their type of collection is different from dressage collection.
In a way, dressage makes use of the idea that getting the back stretching and working is a good thing, not by putting the head and neck low all the time, but by occasionally stretching down the neck, and also by always 'sending the horse to the bridle', so that in a very small way, he's constantly stretching his muscles, even when he's relatively 'up' in his neck. Later, when the horse is collected, the stretching and working has made the back strong, so it doesn't hollow out or invert or 'drop' down like a sway back.
If you want, you could post pictures here and give your evaluation of the picture, and others then could comment. Maybe even pick pictures that most puzzle you.
Keeping in mind of course that no one is ever perfect and that there can even be bad moments in a ride that is good or even great overall.
Even a score of '10' means, 'hey that was pretty darn good' rather than 'perfect'.
But overall, I would really caution you to not get too into critiquing pictures. There is a lot that you can guess or suggest about a picture, and a few things that you can see for sure, but much information, much of the most important information about a ride, you can't see in a still picture.