Choose one of several ways to determine the amount of wire remaining on a spool.
Counting Scale: Measure a sample length of wire, and then weigh the whole spool. You must have an equivalent empty spool in order to subtract the tare weight. This method is not exact, but gives a good idea.
Time-domain Reflectometry: For multi-conductor cable, and especially coax cable, the Harris TS100 Cable Fault Finder can measure distance to end of cable. (Google "Harris TS100" for more information on this product). This product does need a correct propagation velocity, which requires you to measure a known length of equivalent wire.
Volt Ohm Meter: If you have access to both ends of wire on the spool, the low-cost solution is to measure the resistance of the wire with a volt-ohm-meter (VOM) and—based on wire gauge—calculate the length. This solution works on single conductor cable too. See Cirris’s calculator to Estimate the Length of a Wire.