Take a plant(25% of your crop would give you a better ratio for an experiment) and put it in the the dark for two days, take pics before. Then compare them to the others. Pics do not lie. If you notice a dif. then send in some samples to have them checked out.
Plants make up this process during lights out, but I do not think they have the capacity to keep up the processes during lights out for days as they need the light to keep the process going. Happy growing. 👊