Here is a quick rule of thumb to help you if you are concerned.
Basic calculation – (for a quick analysis only)
This calculation is based on a 25% moisture density which may be conservative for our current snow fall. As a rule of thumb, saturated snow weighs approximately 20 pounds per cubic foot.
Calculation: S x 1.25 = P where:
S = Inches of snow on the roof (depth)
1.25 = Weight of 1 sq ft of snow for each 1-inch of depth
P = Pounds per square foot (lbs/sq ft)
Example: If the snow on my roof is 20-inches deep, what would that equate to?
20-inch roof snow depth x 1.25 lbs/sq ft = 25 lbs per sq ft of roof snow load
Source: >Should I shovel the snow off my roof? - CDA Building Department
building.cdaid.org/images/Handouts/Shovel%20Roof%20Snow.pdf
lol. They said if you really want to know quickly just cut out one square foot of the snow and weigh it! :B
pdf is a real eye opener on weight of snow, good read.