The easiest thing to do is to look at the park maps. The icon for the restrooms is a stick figure of a man and a woman.
Here's a link to the Epcot map.
When you get to the park, look around in the general area that the map says the restroom is in. The signs may look a little different depending where you are, but you should have no trouble finding them.
I saw you asking about wheelchairs elsewhere on this board, so wanted to add that almost all, if not all, restrooms have a wheelchair accessible cubicle in them. Be warned that some of them are fairly tight, but I've yet to find one that I can't squeeze my chair into. It's a fairly standard sized manual wheelchair, roughly the same size as a regular rental.