they'll only go broody when they decide its the right time.. mostly when the right daylight kicks in. you can increase that with artificial lighting... keep them mites/lice/worm free. keep them healthy.. not fat nor skinny. fresh water, best to keep them on grass or let them free range a couple hours a day. if you want to see how far along they are.. pick the hens up, and feel around their vent. you'll feel 2 bones (pelvic bones) if you can fit 2 fingers inbetween the bones they are or are about to lay. For bantams im still experimenting to see but im guessing one since their so small. have also heard leaving a few fake eggs in the nest will increase the "want/need" to lay
PS. when it starts to get time for laying i like to feed them some fruit. also will mix up some bread/eggs to give them and stuff like that.