See completion of an area while not in the neighborhood

Unfortunately, as of release, there's currently no label on the World Map or anything else that marks the Asura Gates.

However, in Lion's Arch, the NPCs standing near each gate are indicative of where the gates go to. In the image here, you can see 2 Norn at the left gate to Hoelbrak and at the right gate, 2 Charr at the gate leading to the Black Citadel. So at least there, you can just glance at the NPCs standing at the gates rather than having to run all the way up to them.

Those covered areas of the map count toward achievements (Kryta Explorer, Maguuma Explorer, etc.), but not toward World Completion.

The only things that concern your world completion percentage are the Points of Interest, Renown Hearts, Waypoints, Vistas, and Skill Points in the four WvW maps, and the main world (excluding Southsun Cove and Eye of the North). Dungeons don't count toward map completion, so you don't need to worry about those either.

Unfortunately, the only way to know whether or not you've uncovered the entire map in a particular zone is to thoroughly study your own map. You could also compare your map with one from the Wiki, if you are worried you might have missed something.