I have tried the golf ball deal, not a lot of luck. It has a great deal to do with the day length. And hormones,.... and prayers (I think?)
But this is the time of year that they begin to get the notion, although my second one went broody the 3rd of July, and her sisiter (both buffs) went the very next day, they raised the chicks together. It was great.
However, just in case the Broody Gods are listening, I have a bowl of eggs on my kitchen counter (I write the date on them with a pencil), then every few days, I put more fresh eggs in bowl, and the rest in the fridge. One can keep eggs on the counter for a couple of weeks with out them growing into chicks and with out them going bad. But eggs in the fridge, may not have a high hatch rate. So if one of my girls goes, I will be ready.
Eggs do not begin to turn into chicks until they have been sat on for about 24 hours.
A good broody hen is worth her salt, and once you raise chicks this way, you won't want to go back, it is all the fun and none of the work.