People use different methods to determine when a plant is ready for harvest. Usually you look for the white hair-like things to turn red or reddish brown. Some people will use a small magnifier to take a close look at the trichomes, to see if they've turned amber or some other darker color. You could cut it down when it's been full of bud for a while, white hairs are red and it stinks like crazy.

On sterilized soil, I suspect that all soil is sterilized. However, bacteria is included after the sterilizing process. It would not be a good idea for a company to put out potting soil without killing off disease causing agents first. If a soil is "live," the bacteria was added later. That's all.