How to find out if an egg has a cracked shell before boiling it?
The main reason eggs crack while boiling isn't because they are already cracked, but because of the sudden change in temperature from cold to hot.  The same thing can happen to glass or metal, or practically any substance.  (I've actually broken a glass coffee table top once by setting a pie pan out of the oven on top of it to cool... after a couple minutes, the top crashed to the ground, giving me quite a start!)

Some suggestions to prevent eggs from cracking while boiling can be found here. A summary is:


Let the eggs warm to room temperature on the counter for 20 minutes before boiling.
Put the eggs in the pan before heating the water, and raise the water temperature slowly (i.e. don't set the heat to 'High')
Add vinegar or salt to the water to help heal any small cracks.
Cook the eggs slowly.