My dealer(s) usually have always weighed the weed out in front of my face just because I knew them closely and they know I don't like to be cheated because I spend a lot of money and I can always get it somewhere else. They never weighed it in the bag and only a few times did they have to undercut me which was fine, because I was buying some high quality mids at schwag prices.

Invest in a finger weight. It's quick easy and will give you an approximate value of weight.