There is no guarantee when it comes to broodiness. One can stack the deck by selecting breeds known for broodiness and/or getting stock from well-established lines/breeders (vs hatchery chooks) but mother nature is ever-ready to toss a kink in your best-made plans!
When my pullets approached POL, I first started noticing the occasional oops-egg out in the corner of the run as I was cleaning. Realizing I had at least one layer at that point, I started paying more attention to them during the day. It was VERY obvious that something was going on as a few of the pullets started pacing the run and coop perimeter in a VERY agitated and almost frantic manner. They were bobbing their heads up, down and all around like they were looking for something and they'd try to squeeze into the most impossible nooks and crannies.
When I'd catch a pullet behaving like this, I'd gently pick her up and place her in one of the nest boxes. The first few times she'd just hop right out but after some repetition (and some blocking the front of the next box for 20-30 minutes so she couldn't hop back out) she'd finally settle down and do her thing. After about a week of trying to catch my first few layers in the process of doing what I can only best describe as a young child's pee-pee dance (they know they have to do something, but not sure what and where) and placing them in the nest box, they started doing it on their own. More importantly, the other pullets would catch on and also go to the nest boxes when it was their time to start. There were of course a few laggards that needed some remedial/refresher training when I'd notice 'em doing the pee-pee dance a little too long but they quickly got themselves sorted out.
Other than that, be ready to change over to a good layer ration (or some combination that gets them a balanced diet) when you notice the first egg or two. Starting calcium (layer ration) a little too soon won't hurt them but I've always preferred feeding the entire flock a grower ration with oyster shell offered all the time on the side for those that need it. They know what they need and will only take as much calcium as is required for egg-laying.