Some people have had better luck than me, but... Day 14 of flower is going to be the exact time that you know for 100%.
Bagseed is scary, you can have a plant that shows hairs in late veg.. but then could bust ballsacks everywhere after the pre-flower stretch (day 14 flower).. Good luck!
Bagseed is a result of hermie genetics, so it's a good idea to be prepared, and expect hermie traits.
The ballsacks can be snipped off before they release their pollen, if diligent enough. Happy growing!!