While vistas, POIs, wps, hearts and skill points are listed and easily visible on the map, allowing a player to compare a fully explored map from the internet with his own map, this is not completely true for the concept of "areas", required for certain achievements split in Ascalon, Kryta, Maguuman and Shiverpeaks.
To get 100% map completion you don't need to explore every area, so it becomes hard to tell, in the end, which areas you missed while exploring.
What is then the faster way to see which areas are missing? Is it just a matter of looking manually for "blurs" in the map where you haven't been yet?
Bonus question: Will exploring different areas of the map with different characters on the same account add up in the achievement (like jumping puzzle, for example) or, like standard world completion, you have to explore them all with the same character?