You'll see their combs get large and red very quickly. Seems like it happens overnight. If you don't have a rooster, then the hens will crouch for you. They'll get low to the ground and spread out their wings and wait for you to mate them. It's a very distinctive stance. They'll also get interested in the nests and will look at them and even sit in them to try them out for a couple of weeks. Some golf balls or fake eggs in the nests help pullets know where to lay.
You didn't tell us the breed, so I don't have a prediction for when to look for eggs. Production breeds around 18 weeks, other breeds 22-30 weeks.
Oh, and get ready to switch from chick starter to layer pellets (or add free-choice calcium if you're using a Flock Raiser) when you see the first egg.