How can you tell if an egg is cooked?

For hard-boiled eggs:

Eggs are boiled enough if their yolk is firm and bright yellow. They are boiled too much if the yolk develops a greenish-grey membrane.

Submerge them in a pot of water and bring it to a boil on the stove. Once the water is boiling, turn off the burner, stick a lid on the pot and let it sit for 15 minutes. Then, drain the water, rinse the eggs in cold water, and refrigerate. They'll be perfectly cooked every time. No need to make guesses.

You can tell if an egg is hard-boiled by spinning it long-ways-up on a counter top. If it stays up and spins, it's boiled. If it falls, it's not.

Another way to know if it's boiled:

Get a cup and put water in it then put the egg in it, the egg will float if boiled and sink if raw.