Eggs have to be cracked open and checked for blood spots.  Each individual egg must be checked in a clear glass by the Mashgiach (Kosher Supervisor) to make sure that there are no blood spots in the eggs.  Blood indicates that the egg was fertilised and would render the egg non-Kosher.  Liquid Egg from a Kosher supervised factory does not have to be checked.

Halocha of eggs

All eggs must be cracked and checked one by one for any blood spots prior to use.

If you see a blood spot, dispose of the whole egg. Rinse the checking-cup with cold water.

When cooking free-range eggs cook an uneven number - but more than 1 (e.g. 3, 5, 7) in one pot. This is so that if some eggs are found to contain blood, the pot need not be Kashered provided that the majority was Kosher.

Any number of battery eggs may be boiled.

Eggs cooked in a milk pot cannot be used for meat use and vice versa. It's best to cook them in a Parev pot.

Liquid eggs (with the Beth Din Hechsher) may be used without checking.