There's a book called THE LAME HORSE by Dr Rooney. It's very informative and a good one to own.
Foundered horses tend to lean way back and put weight on the hind end, and once you see "walking on egg shells" you will know what it is. That's what it looks like in the acute, most painful stage.
Anytime a horse "nods" his head excessivly upon putting more weigth on a certain front foot, that can indicate pain. Frequent stumbling can be an indicator of pain sometimes, constant shifting of weight from front leg to front leg (not just stomping flies) or pointing the toe forward at rest.
If there's lameness in the hind end, it's usually more subtle. Watch the top of the hips. The sore leg will have a hip that "drops" with each weight bearing step. The leg may move forward very stiffly, or the horse may hold it up and "skip" when asked to canter.
Reluctance to move, or holding up a limb are clear indicators of something wrong.