co2 and how to tell which week of flowering im in?

Many of my plants are in different flowering weeks, and i was wondering if theres a sure way to check which week its in. I'd say some are in their 4th, 5th, and maybe 6th week of flowering, but im not sure... Will adding a co2 enrichment tank to the room now affect the bud growth at this point? Im pretty sure im getting one tomorrow and just want to make sure its gonna be worth it this grow. Thanks fellas!