Probably have to make a chamber cast with ferro-safe or some such material. Then measure the chamber cast and bore diameter, comparing with a reloading book that shows cartridge dimensions.
I had a Brno action with a heavy target barrel installed that was in a "mystery" caliber. Did the above steps and still didn't find a match. Called a professional gunsmith with my dimensions. He eventually figured out the rifle was chambered for the wildcat cartridge 6mm/.284! I had him pull the barrel and set it back then rechamber for 6mm Remington. Made a beautiful shooting rifle.