Tiny larvae make "window panes" in the leaf:
Mid-sized larvae make irregular holes in the middle of the leaf and move around a lot, typically resting away from their damage:
Large larvae often feed at the top of the plant, where they may eat 1/4 or more of a leaf in one feeding bout (below). They may also damage the flowers and developing pods.
The larvae themselves are very hard to spot. Large larvae will often rest off the plant away from their feeding damage. Tiny larvae are well camouflaged and can easily be mistaken for a leaf vein.