How do you find out caliber of a rifle?

If it is not marked on the rifle, the only certain way is to have a gunsmith make a casting of the chamber. This uses a material known as Cerro-Safe, which melts at a very low temperature, and shrinks a minimal amount when it cools and hardens. The smith will plug the barrel, melt Cerro-Safe, pour it into the chamber, and after it hardens, push it out. The casting is then measured with a micrometer, and the measurements compared to known cartridges.