A 575 Corollary

You would immediately revert to the last form you had assumed via Shapechange, with no other action on your part.

A 576 Addendum

On first inspection, any character is entitled to a reactive Spot check to see through a disguise effect of Polymorph; that or similar form-changing magic grants +10 to the opposed Disguise check, with various Spot and Disguise modifiers depending on the particular form chosen. After that penetrating a disguise takes extra effort.
Quote Originally Posted by Spot
Trying to spot something you failed to see previously is a move action.
Unless the observer makes a move action Spot recheck, they will never penetrate a disguise they failed to see through at first sight.