3 days after the egg has been incubated hold it up to a strong light so that you can see through it. a fertile egg will have the beginning of a embryo and network of veins on the yolk, it will look...
Until they hatch, you can't. Edit added: Very true; so don't buy any of the so called 'egg-sexing' swing magnets and other devices sometimes offered for sale on Ebay!They don't work! If they did...
All chicken eggs are eating eggs unless they have been fertilized and incubated. Even then some people eat them and they are call balut. But to answer your question, almost every egg you have eaten...