code: public boolean isVisited(int tx, int ty)
      {
          return mapHidden.getTile(tx, ty).getNumber() == MapTileFog.NO_FOG;
      }
docs: In case of active fog of war, check if tile has been discovered.
  
  @param tx The horizontal tile.
  @param ty The vertical tile.
  @return <code>true</code> if already discovered, <code>false</code> else.