How do you know if there's a chick in an egg?

I'm curious, how can one determine if there's a chick inside an egg? Is there a specific method or test that can be done to accurately identify whether an egg is fertilized and contains a developing embryo? It seems like a tricky question, as eggs can look very similar on the outside, regardless of whether they're fertilized or not. I'm eager to learn more about this and understand the process of determining the presence of a chick within an egg.