If you will measure the total available length for the spring at full recoil like Scott said, and then measure the wire diameter of the spring and multiply this by the number of coils, and compare the two numbers, you are there. Measuring the spring this way gets you its fully compressed length-- otherwise you're going to need 4 big hands, Z-rated safety glasses, and a steam-powered dial caliper! As long as the spring is not coil bound in full recoil you're OK. If you had to nip a coil or two to prevent it, I don't see a prob.
I do sometimes cut recoil springs for some special applications like for shooting 130 power factor loads out of a compensated .45.
Scott, somebody mentioned a recoil spring tester and you in the same sentence. Is this a product you make? Is it the rod-and-weights thing I saw a few years ago for testing a spring's rating? I suppose with such a took you could easily close the spring and measure it's coil-bound length.... info please, and a pic if you have it.