One frequent misconception I see in gyms is trainers/coaches blaming hip or knee mobility for a person’s lack of proper squat depth. I feel this is often more of an issue of stability / balance / motor control / ankle mobility (see more on this here).
Testing for hip and knee flexion needed for proper squat depth is incredibly easy…have an athlete lie on their back and pull their knees to their chest while pulling on the shins. If their thighs contact their chest/stomach (with back remaining flat) and their calves touch the posterior thigh then you can be sure they have adequate MOBILITY for achieving proper squat depth. If not, we must look at motor control / balance / stability problems (which you can read more about here).