Someone has already asked that - there's the answer I found:
There are several ways but the easiest way is to put the egg into a dish of water, if the egg floats discard it. If it does not float but turns small end up but does not break the surface then it is probably stale. Another way is to "candle" it or shine a very bright light into it and see what it looks like inside. dark spots mean it has either blood spots or an embryo inside and it is best thrown away or boiled up for pets to eat. Small blood spots are not unusual and can be ignored as they disappear when cooked. As with any suspect food keep this in mind.."when in doubt throw it out".
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Bad eggs won't destroy your game. I don't entirely know much about them, but most don't hatch. Try to get rid of it, though. Bad eggs are called so for a reason!
Bad Eggs very rarely, if ever, hatch. They're not really Pokemon eggs. If they do hatch, they hatch into a glitch Pokemon or another Bad Egg.
Bad eggs mess up your game.But at your ownrisk you can hatch a legdndary pokemon.
Discard
A bad egg is a bad egg.