In the map screen you have a nice breakdown of map completion in the current area. Is there a way to see completion of other areas too? (So that you know where to go next)
In the initial release, there was no way of knowing which zones had partial/full completion, or even what the progress was in any zone other than the one you were in.
The only way to know for sure was to go into each zone and check.
However, as of a recent update, the game allows you to see your progress by mousing over the zone name on your map.