How do you tell when the flowers are ready to harv...

When you see your plant fade from green to yellow that is a sign that it is done. I wait until most of the leaves of gone yellow. But it is really a personal preference. You could try and cut some buds while they are still green and compare them with the faded buds with a smoke test.

@Sinktip In the wild the plants will die off gradually leading up to maturity but with home growing the best and easiest method to ensure you are harvesting at the best possible time most of us use a small jewellers type loupe which is a small handheld mini microscope with about 30x magnification. You simply look at one or more of the top buds on your plant for trichome development. Basically what you are looking for is the trichomes to have turned from clear to milky in appearence and when aprox 20-30% of the milky ones progress to an amber colour you are good to go mate .

The buds are at peak when the trichomes are cloudy. Only effective way to determinate that is by using a jeweller loupe, magnifier or other tools to zoom in on the trichomes and verify. For simplicity you can let your pistils turn orange/brown and harvest then but start flushing when you see your pistil 70% orange.