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The technique you’re looking for is called Candling. I used to do this when I made the flu vaccine.
Its basically a light with a rubber cup on it to focus the light into the egg. Works really well.
Here’s a random website that seems to have good info:
https://www.thehappychickencoop.com/the-complete-beginners-guide-to-egg-candling/
Candling will not tell you if the egg is fertilized until Day 4 or so, when the veins start to develop.
Day 4 assuming the egg is at chicken body temperature.
That is, you could be at day 14, and the egg is still fresh and fertile, so long as the eggs haven’t been at chicken temperature.
As soon as they sit at chicken temperature, they start to develop.
A chicken can lay eggs in the same nest for weeks, then start to sit on it, and they all start to develop once she sits on them.
Listen for the tiny screams.
What kind of eggs are you buying that you suspect may be fertilized?
I guess it’s called farm eggs ? , like that eggs you sell when you play Farm Frenzy
Hold the egg up to a candle or light bulb, you should be able to see through most common eggs.
Uh, if you bought them at the store and they’re fertilized then bro something is seriously wrong.