Lay the gun down... Get a cleaning rod and put a patch on it... Insert the patch into the bore just enough to fully seat it into the lands... Near the handle of the cleaning rod wrap a piece of tape around the rod... Place a small mark on the tape to use as reference (notice I said small)... Lay a ruler thats at least 16 inches long next to the cleaning rod... Looking at the reference mark you put on the tape, note where it is compared to the ruler... Push the cleaning rod into the bore (using the handle) and measure the distance it takes for the reference mark to rotate one full revolution... The distance it takes to complete one full revolution is your approximate twist rate...
Example: If the reference mark on the tape travels 7" to complete one full revolution, the barrel is 1:7 twist...
8" to complete one full revolution = 1:8 twist...
9" to complete one full revolution = 1:9 twist...
10" to complete one full revolution = 1:10 twist...