If you have a 7.62x54R case or round handy, you can check which the chamber is. Remove the bolt entirely, and drop a single 54R round or case into the chamber...if the rim is resting against the face of the receiver, than it is probably 8x56R. If the rim is slightly raised from the face, than it is probably 8x50R.
Oh, that rifle is a nice-looking piece of work. I'd say go for it unless the price is ridiculous.