Papi,,,You can determine the twist rate by pushing a tightly patched cleaning rod through the barrel and measuring the distance it takes to make one full turn of the cleaning rod...Place a small mark with a Sharpie on top of the rod near the handle and with the rod in the barrel, place a mark where the rod enters the rifle...Push the rod through and when the first mark makes the first complete revolution, mark the rod again where it is at the same place you placed the second mark...
When you withdraw the rod, measure the distance between the second and third marks and that will give you your barrel twist rate...
You didn't say what brand the rifle is and some were put out with 12 twists and some with 9 twists as the most common, others may be different though, but knowing is important when choosing specific bullet weights...I have a couple of 1/9 twist barrels that function very well with 40-68gr bullets and I have a 1/12 twist that tops out at 60gr for the best accuracy, a 1/14 twist that I used to have was maxed out at 55gr..