If you can't find any markings, I would have it checked by a gunsmith before firing it just to verify what it is chambered for. If it still has the military barrel it is probably
8mm Mauser, but they did make some in
7mm.
Both 7mm and 8mm Mauser ammo is available, (8mm is still available as surplus, but corrosive) but you need to find out what it shoots first.
Not sure about the Czechs, but the Germans marked the barrel near the receiver ring with "7.92" on their K98s, indicating 7.92x57mm, AKA 8mm Mauser.