Summarize this article:

Laminitis is a disease that causes lameness in horses. It was once associated primarily with overweight ponies, but it can affect any age or size of horse.  With this disease, the laminae weaken, which attach the pedal bone to the inside of the hoof. The horses pedal bone detaches from the inside of the hoof. Therefore, the pedal bone can turn, and sometimes, it even pushes through the sole. When it pushes through the sole, it's called "founder." This bond is composed of a non-living layer and a sensitive living layer, much like nails and the nail beds. The detachment happens when this layer disconnects. The early stage of laminitis is called the acute stage. In this stage, the horse has developed laminitis, but the hoof hasn't detached yet.  Pay attention to your horse's mood. He may seem down, and his appetite may be less than normal.  He'll try to avoid exercise, as walking will be painful. He also might be standing differently than normal because he is trying to relieve the pain. For instance, he may push his front feet forward to relieve stress.  Look for changes in walking, such as not being able to turn easily, especially on hard surfaces. Even if he seems fine while walking on soft surfaces, how he acts on hard surfaces can be a dead giveaway. Also, the horse may want to shift from foot to foot all the time. The hoof may grow in differently, creating odd rings. Also, the white might grow out larger than normal in a ring. You may notice flaky hooves or hooves that are crumbly around the edges. Also, feel the hooves, as they may be warm to the touch during the early stages. When your horse is entering the early stages of laminitis, his heart and respiratory rate can begin to increase. The heart rate may move up to 60 to 120 beats per minute, while the respiratory rate may move to 80 to 100 breaths per minute. The subacute stage of laminitis is when the laminitis has gone past 3 days, but it still hasn't detached. Symptoms are actually less noticeable in this stage, but they are the same as the acute stage, such as increased heart rate and changed walking stance. His feet are painful so he wants to take the weight off them. A typical stance is to rock backwards, putting most of his weight on his heels. This makes the horse look as if he's leaning backwards. It is also difficult to get the horse to move. Having found a relatively comfortable way of standing, he's reluctant to move and cause pain.The chronic stage begins anytime the hoof detaches. Watch for the horse wanting to rest. If the horse wants to lay down or rest more often than normal, that can be a sign of a problem. Similarly, if the horse refuses to come out of his stall or is reluctant to walk, that is likely a sign that laminitis has reached the chronic stage.

Summary:
Understand what Laminitis is. Look for early signs of laminitis. Check for hoof changes. Check for increased heart rate and respiratory rate. Check for signs of the subacute stage. Look for changes in the way the horse stands.