Signs of back damage are most often seen as pain and difficulty in riding. Most forms of back pain put extra strain on the back muscles and muscle discomfort can be easily detected. If you suspect your horse has back pain call the vet to see if he can find out exactly where the site of the problem is and if you both still have not found out exactly where the main problem is ask the advice of an osteopath.
Horses that have muscle damage to the back often need at least 6 months off work in order to achieve full recovery. If you do not allow such period of rest the situation will become worse and worse.