Its hard to tell from just looking at it. Do you know about checking trichomes? Look through the harvesting and curing section of the forums if not. its the best way to gauge if the plant is ready to be picked. Its also a good idea to crop the plant over the course of a week. When you take the crowns, the lower branches catch a lot more light and produce more. I have gone so far as to flush for a week, pick, add nutrients for 3 days, fresh water for three days, final flush for two and pick again. It wastes chemicals but you can end up with a few extra ounces off each plant doing it this way instead of cropping the whole plant at once.