A cleaning rod with a bearing handle. Use a snug fitting patch and jag. Mark the top of the rod with a felt pen and push it in till it has made one revolution. Measuer how many inches it took to make the one complete Rev. I would repeat this several times and kinda average the results.
may not give you an exact one but it will be close, use a cleaning patch, or several, jammed really tight in the lands, mark your cleaning rod right at the muzzle and push it down the barrel (making sure it rotates as you do it) until you have made one full revolution. Mark the rod again at the
muzzle and measure the distance between the two marks. Should give you pretty close measurement as long as it doesn't slip too much.
oops someone beat me to it as I was typing mine