I started giving my pullets layer feed around the age of 17 - 18 weeks since some breeds start laying that early.
Signs to look for: The pullets will start investigating the nest boxes - may even sit in one for awhile. When you reach out to pet it, she will "squat." Squatting means she hunkers down with her wings out just a little. It's what they do when a rooster approaches them - it's a sign of submission. They may start doing what's called the "egg song" - which is just a particular way of squawking. It's a "bawk-bawk-bawk-bawk- BAWK!" squawk. Once you hear it, you'll know what it is. Another sign is that their comb will get really red a few days to a few weeks before they start laying.
ETA: Once they start laying you can provide oyster shell to make sure they're getting enough calcium - although layer feed includes calcium. I still make the oyster shell available to them.