The only spanish mauser I've ever handled was in a gun store, and I've never shot one. It seemed like a...mauser lol
What I mean is the construction was sound, and it seemed like a decent rifle. Having never fired one though, and not knowing too much about them, I would figure that your best bet is to inspect that particular rifle.
How does the bore look? How heavy is the trigger? are the screws still in good shape and tight? How is the inside of the stock where it makes contact with the action and barrel? if the rifling is shiny and clean, and everything fits snug, it should be accurate.
One thing though, about surplus rifles and accuracy...Some people I've talked with didn't know that the lowest the sights can be set on many military rifles is often 200 or 300 meters, and because of that its not zeroed properly when you try it at 100m. North American factory ammo and the ammo the rifle was designed to fire often have different ballistics.
The best way to evaluate how a rifle shoots for yourself, if you would like to try is to fire it from a secure rest, take your time, and see what happens.
Hope I can help ya, all the guys here know way more than me lol.