Most barrels do have the twist marked but if this one doesn't you still can find out with a cleaning rod/patch. Put the cleaning rod (without patch) in from the muzzle and let it clear the chamber then put the patch on. Pull the rod toward the muzzle until the patch clears the chamber and is in the rifling a couple inches then wrap a piece of tape around the rod as close to the muzzle as you can. Leave a single tab of tape sticking out or draw a mark on the tape then pull out the rod a few inches, watching for the tab or mark to rotate as the patch in the rifling forces the rod to turn. Continue pulling out the rod until the mark makes exactly one revolution. Measure the amount of rod that was pulled out of the muzzle. Eight inches is 1:8" twist. Nine would be 1:9".
You may have to repeat this a time or two until you get the hang of it.