how to know if an egg is still good

Put the egg in pan full of water
If the egg floats, then you have to throw it away

if it stays in vertical, then it's better you eat only if cooked

if it stays on the bottom of the pan and in horrizontal, then you can enjoy it as you like, because it is fresh!

See also http://www.comefaretutto.com/come-capire-uovo-fresco/