How do you get to know when its flowering?

Hi @fabhz! You'll notice a dramatical growth, the plant will stretch, and the spaces between nodes will increase, it's the beginning of pre-flowering. Height can double/triple in a few days! Then you'll see white pistils/hairs : plant is blooming/flowering, and will put all its energy into buds production. Hope this helps, happy growing!

White hairs (pistils) will appear and rapid growth will happen (you can see the white hairs on top of your plant starting to show.