Usually pretty easy to tell. Roll marks won't be as sharp, finish almost certainly will be different than originals. But I have seen some "minty" SP1s and even some guys with some still NIB examples from the mid 70s that they have never fired. Most people won't try and fake a perfect condition SP1, it's too easy to spot and then you get nothing. And most guys are looking for one with just a little honest wear and patina.