I'm no mechanic, but the fingertip test is one method I use. Go along about every 2 inches and compress the cushion nose with your index finger tip. The cushion should consistently compress around 3/16". If there are hard spots on the cushion where you can't compress the nose that amount, or mushy spots where the cushion may compress more or not immediately rebound after you let go, then your cushions need replacing. Rolling a ball against the cushions every few inches is another method, but don't confuse a thud that may be caused by a cushion becoming loose from the subrail, as the cushion could simply just need to be reglued.
Dribbling a ball along the rails every inch or so will help you detect dead rails as you can hear the difference. Does it rebound easily or go thud is basically what to look for. Also, if the rails are loose or not secured right they can seem dead.