They are also clearly indicated on the park maps.
On top of my head:
Main street: next to city hall, opposite of city hall, and between the arcade and the plaza restaurant. Also on both sides in the middle of the arcades.
Fantasyland: At Pinocchio's restaurant, next to the "tower" on the walkway to it's a small world
Discoveryland: Next to Autopia (there are two doors/ entrances, one is less used/ cleaner as the other one) and in the show building of the Lion King, near the train station
Frontierland: when entering Frontierland at the Indian Village (right side), between Silver Spur Steakhouse and Phantom Manor, opposite to BIg Thunder Mountain (between the Mexican CS and the Cowboy Cookout thing)
Adventureland: next to Colonel Hathi's (or whatever it is called) restaurant and next to the Agrabbah Cafe.
I don't know them too well at WDS:
There are some in Studio 1, at the end on the shop-side
There are some at the entrance, next to guest relations
There are some next to the ANimation Building when walking to the Backstage Tram Tour
How pathetic am I that I know where the restrooms are?
