Saddle Too High? Watch The Hips

Set your saddle too high and you can't reach the bottom of the pedal stroke — so your hips rock side to side to get there, once every single turn. From the side you'd barely notice it. From behind, you can't miss it.

Too low is the opposite problem: the knee never opens up and you pedal in a squat.

What gives it away is the hips, not the legs. If they tip left-right in time with the pedals, the saddle is too high for that rider — and it only shows from behind.