Unless it has been altered, it's a pretty good bet the caliber is 8x57JS. The VZ24 was available in 7x57 and 7.65x53, but those are pretty rare.
Of course, many milsurp rifles have been reworked, re-barreled or re-chambered over the years, so if there is doubt, have a gunsmith run a chamber cast to make sure.
Jim