My personal way of knowing when to pull is:
1) Wait for the plant to look so sparkly, it has a glassy type shimmer.
2) Feel the buds, ripe buds are harder.
3) Wait till the buds swell over the withering pistils, it will look like the pistils have receded into the buds.
I used to go by the pistils changing colors or by the scope, but there are too many strains where judging by either of those ways is not accurate IME. Hope this helps.