I don't know that's there's an "official" method.
Personally, I try to grab clippings from half-a-dozen spots to study. IF the majority were ready, I'd probably grab the plant as soon as it needed watering.
I think I recall reading a Fdd post stating that they seem to ripen from the bottom up?
I'm not sure if that's each bud, or the whole plant?
I just got the Radio Shack one and it's a whole lot better than nothing (my husband and I were just looking at everything last night saying "WHOA!....WHOA!"). It certainly saved me from harvesting too early!