Well, the best I can tell ya, is to wait till
all the pistils have turned brown, then wait a few more days, or a week. Some strains keep pumping out white pistils, so this may be tough to do, but many Indica-doms don't. At any rate,...almost always, you should resist and wait longer, because I know how tempting it is to chop asap, which is always ahead of schedule. Basically, they 'look' done when they are truly done, and even the buds take on an amber kind of look to them, after the trichs have turned. Hard to describe, but you'll get the hang of it, eventually.