Courtesy of TrueAchievements:
Here is a screenshot showing you most of the locations available for discovery within the game. Spending time thoroughly searching every corner of the map will net this one naturally, otherwise you can use this map as a guide to help cut down your exploration time. I personally unlocked this trophy near the end of searching Detyre, after finding every possible location in both Dalentarth and the Plains of Erathell.
Please note that you can track this one under the "Stats" menu. Also of note (though I cannot confirm myself) that DLC locations should also count towards this achievement.
You will know you have actually "found" a location when, as you approach one, a prompt will appear on screen saying you have discovered such-and-such location. You can confirm this if a fast travel icon then appears on your World Map.