Typically, we meteorologists look at the amount of moisture available in the atmosphere to fall as rain. Then we look at the temperature profile from the ground up to the clouds to see whether it will support all snow, or if it will be mixed precipitation.
Once we decide it will be all snow, the next step is to figure out the liquid-to-snow ratio. For example, would an inch of rain in this storm equal 5 inches, 10 inches, or even as much as 25 inches?
Then you multiply that first number of how much liquid the storm could dump if it were rain by the last number of the liquid-to-snow ratio to get the estimated snowfall potential. For example: Three inches of rain at a 1:10 ratio could yield 30 inches of snow.
In the case of the current storm affecting New England, it's even more complicated. Forecasters also have to worry about the snow "banding" (in summer we call it "training"), which is when the same locations see heavier precipitation throughout the day while surrounding areas might see lighter precipitation or breaks in it.
As you can imagine, "how much snow?" is the hardest part of the forecast - no matter the location.