The best place to start is with the barrel or rifle maker. Sometimes this information is not available. One method of finding the twist rate of your barrel is by using a cleaning rod.
Put a dry, loose fitting patch (so it doesn’t get stuck in your barrel) on a jag and put your rod into your barrel from either the muzzle end or the chamber end. (Note: When putting a rod into the muzzle end, be careful not to damage the crown/muzzle).
Move the rod back and forth to verify that the rod spins freely as the rifling turns the rod.
With the rod in the barrel, make a mark on the rod with a Sharpie at the point where the rod enters the muzzle or the action.
Near the handle of the rod place a piece of tape on the top of the rod. Push the rod into the barrel.
You will see the piece of tape travel one full revolution as the rifling turns the rod.
When the tape is back to the top make another mark with your Sharpie on the rod where the rod enters the muzzle or the action.
Pull the rod out of the barrel and measure the distance between the two Sharpie marks.
If the distance is 12 inches, you have a 1:12” twist barrel. If the distance is 8 inches, you have a 1:8” twist barrel, and so on. For best results, repeat this process two or three times. If you find the measurement is 9.2 inches or 9.8 inches, you can shoot any bullets that are recommended for a 1:10” twist. You may or may not be able to shoot bullets that are recommended for a 1:9” twist.