So is it still in the box or can you see both ends of the cable.
So almost all ethernet cable have the the EIA/TIA markings that show the cable has paid their license fees.....well unless they lie. There should be a foot marking at least on cable used in the USA. On boxes of cable I forget if it starts at zero and then goes up to say 1000 ft or what ever or if it starts at 1000 and goes down. I think it starts high and goes down so you know how much cable is left in the box.
If you can see both ends of the cable you should be able to see the markings and subtrack them to get the length. Most times they are marked every 2 feet.
If you were a pro you would have a fuke meter that will give you a pretty good estimate of the length of any cable.