I remember working at a photo lab here in Minneapolis, which is now out of business, but they had an excellent line for film processing and printing. While working there I remember being impressed with the level of quality control, and viewing of films on light tables with loupes, in order to insure the best quality.
Test strips with densitometer readings about every two hours on all machines, and visual inspection of every sheet that came through, and select rolls from different batches. Obviously we didn't have the manpower to inspect everything, but we had very few complaints from the professional studios that used us; I can only remember two incidents, one which was resolved peacefully and the other where the photographer didn't admit that it was probably his fault that his 4x5 sheets were fogged.