RSArea areaObj = new RSArea(SouthWestTileX, SouthWestTileY, NorthWestTileX, NorthWestTileY);
RSNPC npc = npcs.getNearest(ID);
RSTile npcTile = npc.getLocation();
if (areaObj.contains(npcTile)) {
    //dostuff
} else {
    //dostuff
}