After looking for an easy way to weigh hives, I found some interesting instructions for converting a hydraulic jack into a scale of sorts. The result is a jack with a pressure gauge, the pressure indicated on the gauge is proportional to the weight. It takes either a calculation or a lookup table to convert the pressure to weight in pounds.
My hives are on stands that I can easily put the jack under, I weigh the two ends and add the readings together. It is not super accurate, my guess is that it is probably within 10 or 15 pounds. But it is way closer than guessing, and weighing a hive only takes a few minutes.
Here are a couple of links to instructions for converting a jack. I am sure you can find more if you do a web search.
I found the pressure gauge and the brass 1/4" to 1/8" elbow at Fastenal for less than $20.