If you had any totally empty spools of the same brand filament you would be able to find the weight of the spool. Then on any other same brand filament, you would not need to take the remaining filament off the spool. Just weigh the whole thing then subtract by the weight of just the spool.

This could be slightly inaccurate as spools weight could change, but I would think it would be almost none if any. I have a bunch of empty spools lying around that I may weigh to see if they are different weight.