The best thing to do is check the trichomes. Get a handheld microscope from Radio Shack for $13 or get a good jewlers loop. If some of the trichs are turning amber or brown the plant is ready.

If the trichs on the lower buds are not ready then you can try growing them a little longer to see if the trichs on the lower buds reach the proper maturity. I personally have not had much luck with this but I know folks who claim they have.

Usually when I see some of the trichs are ready all the buds on the plant are ready. I grow outdoor.