3 signs
The light green growth at new tips
The clustering of the flower
The presence of pistils
They are in flower. Most if my autos see first pistil at day 34
Your plant looks really really dark. I'd consider cutting out the nutrients for a week possibly more see if she clears herself up a bit. The excess of nitrogen doesn't help during flower and I think I see pistils on dat there plant so she looks like she is intend starting to flower.
Look at my grow diary. The hairs that are shooting out all over are your clue that flower has started. Use veg nutes till after the first 2 of flower then switch to flower. Happy growing!