moment.tz.zone(name); // Zone or null

To check if a zone exists, use moment.tz.zone. It will return the Zone if it was loaded and null if it was not loaded.

moment.tz.zone("UnloadedZone"); // null
moment.tz.add("UnloadedZone|UZ|0|0|");
moment.tz.zone("UnloadedZone"); // Zone { name : "UnloadedZone", ...}