The optimal way is through the trichomes. Get a phone microscope online and you can take nice photos for your diary right away. I harvest when 10 to 15% of the heads are amber-colored, but that's a matter of taste. They should be milky and no longer clear; when all are milky but not yet amber, you have the highest THC content. If you don't have access to a phone or pocket microscope, you should pay attention to the hairs. At least 80% should be brown. But that's very inaccurate; some genetics are not finished even though almost all the hairs are brown. Either way, the bloom in the picture is not ready for harvest yet, so you'll have to be a little more patient. Best regards, I wish you a tasty harvest.