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.
Brewed a Pale Ale with some leftover hops I had from last batch. Sadly I accidentally replaced (due to it being out of stock) ~60EBC crystal with a ~110 EBC crystal, so it’s quite red. Then the BIAB bag broke during sparge, smashed down into my wort, splashing about 40% of it out onto the ground. And myself.
So we’ll see.