It's good to pull a bud or two throughout each last weeks of flower in stair steps. You can get a better angle on harvest time and the sweet spot for a particular strain. I have always ran things 10 to 14 days past the recommended flower times. Sometimes longer.
I never start flushing prior to the end of recommended flower time. You will notice the plants basically start eating themselves through the withered leaf and the buds will develop their final hues. If your phone has a good camera setting, you can choose the highest resolution photo mode and zoom in on your trichomes.