The best way to tell is to use a jewellers loupe, magnifying glass, or a high Rez photo (zoomed in) to examine the trichomes. Clear trichomes are immature, and will become a milky white and then amber color.
I've heard of people harvesting with anywhere from 20-80% amber trichomes (the rest being milky) based on preference and genetics. I'd say as long as you aren't seeing clear trichomes, which you shouldn't be at week 9 flower, you should be okay to harvest.