Their pelvic bones are directly under their vent. There are 2 bones. The space between the bones should be 2 fingers (vertically) in order to be laying eggs.
It may take them a few more weeks, hard to say, but usually pullets begin to lay at 20 weeks. High production birds tend to start earlier (16 weeks) and some breeds start later, at 25+ weeks. That being said, once they have started laying for about a week or two they should start to lay daily...