Look to your trichs. Get as close a pic as you can, under white light. Once the majority of your stamen(pistils, stigmas, hairs whatever you want to call them) have colored and receded back toward the calyxs, it's time to watch your trichomes. They'll start standing straight out and clear, then the cap will swell and start to turn cloudy, then they will start to collapse, leaning over from the weight of the cap and will start to turn amber. I personally harvest when I'm between approximately 5&10% amber. Your test samples don't need to be large but they should come from multiple points on the plant in question. I've generally found that once you're pretty sure you're done, you've got a week.