Even if your attic is well-insulated, a fast method to see if you need more is to look down the length of your roof. More insulation is needed if your existing insulation is slightly above or level with your floor joists (i.e. your joists are clearly visible). Adding extra insulation may not be cost efficient if you can't see any floor joists due to the thick layer of insulation covering them. Even distribution of insulation is critical. Low spots can occur if too much insulation is placed in the center of the attic and too little around the eaves. If your attic insulation is equally spread and covers your joists, you should be good to go.