Start with the published weight
Every cable construction has a published weight per thousand feet, and most manufacturers print it in the catalogue or on the tag. Net weight divided by that figure, multiplied by a thousand, gives footage within a few percent — close enough for a review and close enough for a buyer to commit to a pickup.
Weighing without a truck scale
A platform scale under a single reel works for anything a forklift can lift. For larger reels, weigh the loaded trailer at a certified scale, unload, and weigh again. The difference is the cable plus the reel, so subtract the reel weight from its stencilled figure or from a typical value for that flange size.
Counting turns as a cross-check
Count the turns visible across the top layer, measure the barrel and the outside diameter, and use the average circumference to estimate the top layer's length. It is rough, but it catches the case where a reel has been cut and re-tagged with the original footage.
Say which method you used
State the method and the number together — for example, 'net weight 1,840 lb, which is about 1,150 ft at the published weight per thousand'. A buyer can then check the arithmetic rather than trust a bare figure.