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.