To make a definitive diagnosis, your veterinarian may use a gastroscope. A gastroscope is a long endoscope, a device with a camera on the end that allows your vet to see a close-up view of the stomach lining. Your horse will be unhappy to learn that a gastroscope is inserted through his nostrils. It is not painful, but it's not exactly great either, so your veterinarian will probably tranquilize your horse before the procedure. A gastroscopy takes about 20 minutes.
There is a fecal blood test now available. According to its manufacturers, the results of a Succeed FBT® reveal "…occult fecal blood from anywhere along the GI tract, distinguishing foregut from hindgut sources." If your horse is suffering from a bleeding ulcer -- but be aware that not all ulcers bleed -- this test is a convenient step towards diagnosis and treatment.
To treat equine gastric ulcer syndrome, reduce the amount of acid being produced, physically coat the stomach to protect it from acid and/or buffer the acid that is being produced.