One obvious sign of lameness is a rhythmic bob of the head as a horse walks or trots. But pinpointing which leg is sore can be difficult.
The bobbing motion is produced when a horse uses the mass of his head and neck to shift his weight away from his sore leg with each stride. In general, if the horse's head bobs more upward than downward, the lameness is likely to be in a front limb: The head rises when the affected leg bears weight and dips when the sound leg is on the ground. If, however, the horse drops his head dramatically with each stride, the source of the lameness is more likely to be in a hind limb: As he trots, he drops his head when the foreleg diagonal to the sore leg is touching the ground.
To better identify the limb that hurts, have a friend jog your horse over a flat, smooth surface while you stand about 20 feet away, perpendicular to their path. Ask your helper to leave the lead line slack so that your horse's head is not restricted. As the pair jogs past, keep your eyes straight ahead and let them cross your field of vision, keeping the entire horse in perspective. If you follow the horse with your eyes, you may focus too closely on a single leg and reach the wrong conclusion. You may need to watch the horse pass several times to determine which leg is affected, and mild lameness may be noticeable only when the horse is turning.
Share your observations with your veterinarian when she arrives to examine your horse. She will do a more thorough diagnostic investigation to pinpoint the source of the trouble.