I'll just reaffirm what others have said. You need to check out the resin glands. Take a piece of the popcorn bud off, wrap it up in a baggie and head to your local public library. In almost all libraries they have a periodical and back issue section where you can read old newspapers. Almost all of them will have a magnifying glass you can use. Just take it to the bathroom and take a look.
Harvesting too early will mean you won't reap the full benefit of your crop, and the THC potential won't be at its highest. Harvesting too late will give a harsher element to your smoke. Really you're at the most important time. You certainly want the best out of your crop don't you?!?
Hope its everything you've dreamed about, mate!