The first thing I do before even touching a horse is a visual assessment. I do this by watching the horse move and checking his conformation. I look for lameness and unevenness in stride, ease of going and overall behavior.
This visual assessment can tell me a LOT even before I am to lay hands on the horse. If a horse has been sore and compensating for this he will have developed muscles unevenly, may be lower in his hip (which indicates a pelvis rotation), may be resentful if asked to move out or display other characteristics that tell me immediately there is a problem going on.
Not always is the problem chiropractic (though the majority of the time it is) but injury and training can also affect soundness and attitude so the next step of a thorough hands on feel is important. I physically run my hands over the horse's body, looking for asymmetry, muscle soreness and tight spots. Between the visual and hands on assessment I have a very good idea what is going on and whether it is chiropractic related.
The next step after identifying the problem area(s) is the adjustment. In most cases after an adjustment the horse has immediate relief and promptly starts to "lick and chew". This lets me know that I have relieved their discomfort in that area and the horse is able to finally relax those muscles.
After the adjustment, the last step is a massage of the sore muscles with a soothing liniment. This not only feels good to the horse but allows the muscles to relax so the adjustment can stay in place.
All my treatments are always two appointments. The initial treatment above and then a followup to make sure everything is still good.