But casting is more fun.
I have a $10 ebay micro-scale that has a 'grains' option built in. It's a fairly easy process. coil up the first 30ft and put a twist tie on it. weigh it and subtract out the weight of the twist tie (2-3grains). I usually tare my scale with the twist tie.
You have to be careful with how the rest of the line lies off to the edge, but it is close enough. It isn't a precise science. Originally it was measure the first 31 feet and subtract out the first foot since that was often level and designed to be cut back after you tie a nail knot on. The actual ratings are a range and depending on the taper of the line.
Umpqua makes a hanging line scale that makes sense if you have a bunch of them. I think it goes for a bit over $20.