Have your veterinarian perform a lameness exam to aid in diagnosing cause of lameness.
A lameness exam is a great way to get the diagnosis of your horses lameness started. The lameness exam is often broken up in to multiple steps:
Physical Examination: visually examining the horse of any visible signs of lameness, conformation defects, palpation and manipulation to view heat, pain or swelling.
Examination in Motion: examining the horse in its 3 gaits (if horse is capable of doing more then a walk). Often done in hand or ridden.
Flexion Tests: stress particular areas and structures in the horse to see if they show any lameness or worsened lameness. Flexion of a joint is often performed and horse is immediately asked to trot away.