Seat Height — How hard can it be? by Steve Hogg
Single most succinct and insightful statement about fitting:
Focus on the velocity of extension of the rear of the knee under significant load. That velocity should be constant, if you see even the tinniest flicker of acceleration at the rear of the knee before the bottom of the pedal stroke, then the rider is losing control of the motion and is too high; at least on that side.
Look at your customer long and hard, especially for movement of the pelvis.
If your fitter is only viewing one side of your body, call a halt and go elsewhere.
Perspectives on Fitting by Steve Hogg