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.
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
Pseudo thinks the supermarket came before the egg or the chicken.
Uh, if you bought them at the store and they're fertilized then bro something is seriously wrong.
Hold the egg up to a candle or light bulb, you should be able to see through most common eggs.
If you care, shouldn't you check before boiling them?
I usually don't care but that time while boiling, the egg exploded something that looked like feather , so I had this question I asked , but when I cut it I found it's a normal egg
Candling like others have said but also in case you didn't know, it is safe to eat a fertilised egg. Obviously if the chick is developed it'd be quite unpleasant but in the early stages I've heard it tastes the same as unfertilised.
Apparently duck eggs (but sometimes chicken eggs) with a developed embryo is a delicacy in some countries referred to as Balut eggs.
Just don't eat eggs
