I've got a chicken which has being laying eggs for a while.

[Rumple]

But how can I tell when the eggs are chicks or just eggs?

 

[nuthatched]

Do you have a rooster? If you crack an egg into a bowl, you'll see a white spot, if it's a Bulls eye, it's fertile, just a spot, infertile.
A fertile egg doesn't start developing unless it's sat on or incubated for 20 or so hours
Then, you can chine a flashlight into the egg to monitor development

 

[ThinMintTheChicken]

But how can I tell when the eggs are chicks or just eggs?

An egg cannot be a "chick" until it has been incubated. Unless you have a rooster or if she is broody or you have the eggs in an incubator, then the eggs are "just eggs"