Easy to determine the twist and that is probably the biggest issue when it comes to picking what projectiles to use.
Take a cleaning rod with a swivel handle. Put an oversized brush on the end or a brush with a patch over it so it is tight in the barrel. Insert the rod into the action until well in to the rifling. Mark the rod, push it in until the rod has turned one revolution and mark the rod again. Pull the rod out and measure the distance between the marks and you have xx:1 twist.