Measuring the levels of plant hormones such as gibberline, ABA and cytokinin(CK) could be useful for determination of flower induction time. Usually, The level of gibberlines (specially GA7) is reduced during flower induction and the levels of ABA and CK is increased. Also, in some plants polyamines have a high level during flower induction. The levels of sucrose could be suitable as a marker for flower induction. Moreover the levels of RNA is increased during flower induction. You can extract RNA and determine the concentration of it. there are many genes that are involved in flower induction that is different in the plants. I have attached two papers for you that i think these paper could be very useful for you.