Invest in a jewelers loupe if you dont have one already. They are very inexpensive, like a few $s on amazon. Then its all about inspecting the Trichomes. Every one is a little different in preferences and what you want out of her. I generally go by the following rule of thumb:
Clear Trichomes - Immature and not full potential (flavor & potency) More jittery/anxiety effects. ( strain depending)
Cloudy/milky Trichomes - Fully mature and at there highest flavor and potency. More heady and energizing ( strain depending)
Amber Trichomes - Starting to decline in Flavor and potency. More body and couch lock effect. ( strain depending)
I like my medicine more for sleep and pain so I like to wait for around 80% cloudy & 20% Amber. I would say a full proof way is mostly cloudy with about 5% Amber. (Everyone is different. Need to mess around and find what you like)
Also make sure you check all buds, tops and lowers and check the trichomes on the flower it self and not the leaves. The leaves tend to mature quicker than the buds. Hope This Helps! Happy Growing!!