Photoperiod plants enter flower when the light cycle, aka the Photoperiod changes, this is how they got their name. normaly this is anything less then 12hours of light.
Outdoors this happens when the longest day of summer is past and the nights start to become longer, however if your closer to the equator it is when the plant reaches sexual maturity. anywhere from 5-10 weeks. as the days are always aorund 12/12. think of it like puberty for plants.
The signs you will see are small white pistils coming out of the nodes, the spots where leaves grow out of. This is of cource if they are female. Male plants will not grow pistils but will grow pollen sacks or as a lot of users call them "balls" Should the plant have both balls and pistils, they are a hermi. containing male and female parts. this is not a desired trait and best to kill the plant.
With is information you should be able to tell now if it is entering flower or not. Also use google image search to get a visual of pistils and balls. Can be confusing at the beginning but it gets very easy with time.
Good Luck!
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