We can say there are too many possibilities in horses legs to identify problems. From ground to more up: Hoof, fetlock, suspensory ligament, superficial flexor tendon, knee. These are the most common areas to find the problems of horses legs.
Only in hoof can be several problems, hoof crack, laminitis, hoof thrush, hoof abscess and navicular syndrome for example.
Hoof crack, hoof thrush and hoof abscess are more easy to find, but with laminitis and navicular syndrome you may need some professional help.
A fetlock can be swollen or not, and even though it can be painful. If there is no wound causing the swelling, the problem is inside the fetlock. There can be a stress because of too hard training or hard ground for example.
I have noticed that many people can't realize this: They think if can't see any swelling, can't be any pain. And then they try to find the problem from somewhere else, blaming some other part of the horse causing the problem. Or worse, accusing the horse he does not want to co-operate.
Ligament and tendon problems can be seen very clearly when they occur. There is a lot of swelling, sometimes so much you can't be sure what is the problem. By checking horses legs everyday visually and by hands you can see and feel if these problems are starting.
Knee problems are not so easy to find out. You may see some stiffness when the horse is moving his front legs, or he is dragging his opposite hind leg.
Example: When a horse has pain in right front leg, he wants to put his head more to the left and push the back more to the right to help the right front leg to move forward more easy.
To start to know your horse better, I recommend you check your horses legs every day. The best way to do it, is checking legs with hands. You can feel if there are any changes like: swelling, heat, wounds, some kind of bump you haven't notice before.
Some other tips about horses legs: How to locate at least the leg where the problem is, continue reading the article here: http://www.horsehoofandfootproblems.com/articles/horseslegs.html