With a 5x magnifying glass you can see the trichomes on the leaves of the buds. (not the hairs, those are called pistils) If blind like me, perhaps a 100x pocket microscope from Radio Shack. $20 last I saw.
The trichomes look like a clear stick with a round ball sitting on top. Like a Tootsie Pop, but clear.

Anyway, as the plant matures, the ball on the trichome will start to turn cloudy or milky. You'll likely want most of the trichomes milky before harvest. Milky is usually a stronger sedative effect and smoother flavors.
I let mine go till fully milky. Suits my needs just fine. :thumbsup:

I'd ditch the foil. Promotes heat retention. White closet walls do a better job anyway. (flat white paint)