Summarize:

Your horse may behave differently than they usually would if they're in pain. For example, a horse may become cranky when ridden, charge at jumps, refuse fences, or buck when they were previously mild-mannered.  A change of character such as trying to bite an owner when grooming the back end, bucking, or general bad temper, can be a sign of pain. Your horse may also demonstrate pain during the tacking up process. Another common presentation is that the horse does not work to its full potential. It attempts to limit the discomfort by not exerting itself, which could mean it:  Doesn't move as quickly or easily. Doesn't reach normal height when jumping. This phrase means that your horse tries to take weight off its hind quarters and shifts its center of gravity forward. When it does this it places more weight on its front legs and moves in a more labored manner because it takes more effort to lift its front legs.   When riding the horse have a friend stand parallel to it and video its movement. Look for the horse lowering its head to counterbalance the back end. Look to see if all the legs are taking equal length steps or if one leg is taking shorter strides than the others.  When riding the horse have a friend stand a safe distance behind the horse and take a video. Look to see if the hips move up and down symmetrically. A horse with a sore back leg will try to protect that leg with the result that the hip moves less. For fluid movement, the horse uses the power in its back end and bunches its hind legs beneath it to move forward. If the horse associates pushing off on its hind legs with pain it will be reluctant to do this, and will most likely move more slowly than normal. You may be able to feel this easily when you are riding your horse. Jumping requires the horse to shift its weight backwards and place a considerable extra load on its hind legs. If soreness or pain is present, it may try to avoid this discomfort by not fully using its muscles to propel itself upward. Your horse may lose height early, which means it will knock against jumps it used to take with ease. For example, you may notice that your horse knocks down fences during jumps. Landing after a jump involves tucking the hind legs beneath the body. This provides spring to push the horse forward onto its next stride. When your horse has a painful hind leg, it may slip and land awkwardly. Hock pain or general hind end discomfort alters the way a horse stands. It tends to shift its weight to minimize stress on the sore leg. Some things you might notice include:  Resting one hind leg while standing. Standing with the sore leg tucked under its belly so that the hock is straight and the leg does not have any weight on it. Standing with one leg on a large mound of shavings to elevate it. Pain alters the way the horse moves, which is referred to as its "gait."  Hock and back end pain tends to make the horse "mince" or take shortened strides with its hind legs. It transfers weight onto his forelegs, which gives him a hunched silhouette with its hind quarters tucked under and head carriage low.  Because it hurts to flex the joint, the horse may not pick its leg up cleanly, and may have a tendency to stumble. A useful tip is to walk and trot the horse on sand so that you can trace its hoof prints. The sore leg tends to move towards the midline, rather than following the line of the matching front leg. If your horse's hock is injured, it may have a hard time walking backwards in a straight line. This is because the sore leg takes shorter strides, so the horse naturally moves in a curve to the affected side. If you notice that there is a loss of muscle mass over the thigh and hip of the affected leg, your horse may have a problem with its hock. This loss of muscle mass is a result of "disuse atrophy", which means that the horse has been protecting that leg and underusing it. When muscles do not get used, they can begin to waste away.  Be aware that disuse atrophy can arise because of pain anywhere in the limb and does not localize the discomfort to the hock. If you're sure your horse has a mobility issue, it is a good idea to call the vet to give the horse a thorough check over. A veterinarian can perform tests to isolate the source of your horse's pain to the hock. They can do a thorough lameness exam including flexion tests, nerve blocks, and X-rays if needed. The veterinarian will also look for other tell-tale signs of discomfort such as head bobbing, unusual foot placement, shortened strides, and weight shifts.
Check for behavioral signs of pain. Consider whether or not your horse is working as hard as normal. Notice if your horse begins riding heavy on the forehand. Take note if your horse is not engaging its back end. Keep track of your horse's ability to jump. Note any challenges your horse has with landing after it jumps. Look at the way your horse stands. Assess whether your horse's gait has changed. Watch for symptoms of disuse atrophy. Contact a veterinarian to further the assessment.