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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 you didn't put them in the fridge?
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.
Eloquently put