Actually, I have a similar question of my own.

I have been smoking for a few years now but I have yet to have a dealer weigh it in front of me. I don't really need to because I can eyeball quality/quantity well. I was just wondering; when it does get weighed, do the do it in or out of the bag, and if it is in the bag how do they take the weight of the bag into account? Just weight the bag lone first?