The differences are samll from our human point of view. It is always difficult to tell whether a Mauser action is large ring or small ring or even a coveted magnum action just on gross visual inspection, say if a single action were just sitting on a table and you could not inspect it very closely.
Originally, the pre M1898 Masuers (89, 91, 92, 94, 95, 96) were designed for cartridges of about 40,000 psi pressure and the recievers had a front diameter of about 1.3".
When the 98 Mauser was designed, it was realized that pressures up to 50,000 psi and greater could be safely achieved. The receiver diameter was then increased to 1.41", an increase of .11" over the previous standard of 1.3".
Now, to our human eyes this is not much, but it actually does increase the strength of the reciever by a considerable amount. The 98 Mauser reciever is said to be stronger and more robust and this is true in most cases.
Despite the new standard, a good number of 98 Mausers were in fact produced in the old small ring size. This was beause the the old calibers were still popular and because people felt that the old size was good enough and lighter to boot.
The same problem occures when trying to spot magnum actions. The difference between a magnum and a standard Mauser bolt is less than 1/2", 6.4" vs 6.77" (.37"). The whole reciever is longer by about as much. This extra length allows the magnum to handle cartridges of the 375 H&H length. Some magnum action are obvious becasue they have "Square Bridges" and extra big magaxines, but many do not. I dream of finding a magnum action that has been misidentified as a standard action with a corresponding lowered price! Hasn't happened yet.
The answer to your question is multiple:
1. Most people do not know the difference and do not care to learn such esoteric rifle lore. It is of concern , mostly, to people who are rebuilding old Masuers in some way.
2. Of the people who do know something about this, the majority just know that such and such a model of Masuer is large or small ring becasue all of that model are that way.
3. The only other way to know for sure is to get a book like Jerry Kuhnhausen's "The Mauser Bolt Action" (which I used here) to look up these dimensional differences and measure the action in question at the critical points.
