If the horses head bobs while trotting, the head will go down when the leg in pain hits the ground.
Watching from behind: the hip that looks higher is the side where the problem is, since the hip will come up to relieve pressure.
A hind leg travels inside: There could be hock pain, low back pain, or muscle spasms of the semimembranosus.
Throws hind leg outwards: There could be pain in the stifle or hip, or spasms in the tensor fascia latae.
The front leg travels inside: there could be knee pain, or tight pectoral muscles.
The front leg arcs out: there could be back or shoulder pain, or spasms in the spinatus muscles. (see first diagram)
Short stride in front: spasm in the triceps.
Short stride behind: if it is on one side, there could be a problem with that hip. If both hind legs are short and tight, there could be a problem with the sacroiliac joint, or the muscles surrounding that joint.
Many of the problems described here can be resolved through massage, cold laser, and/or chiropractic treatment.