To determine the best data source for reverse geocoding, first attempt to determine if the point; for example, a mobile device, lies within the United States or Canada. This can be done outside ArcWeb Services by viewing the location of the x,y coordinate. If the country location of the point is successfully determined, select the appropriate geocoding data source, either one that ends in .US (United States) or .CA (Canada).
If the location is on or near the border, or the country location cannot be correctly determined, use Spatial Query Web Service's method findFeaturesByPoint method with the data source ArcWeb:ESRI.AdminAreas.World to find the country location of the point. The result that has a distance of 0 is the country that the point is within.
With v2, use Query Web Service's method pointQuery with the data source ESRI.AdminAreas.World