.
Fast/simple - look at the LH side of the receiver.
If it's even from the front @ the barrel, all the way to the rear - it's a Small Ring.
If the front/receiver ring is larger, causing a step down along the side there, it's a Large Ring.
Another way - All Large Ring's (all M98's) cock-on-opening; while most Small Rings cock-on-closing - the exception being Small Ring 98 Mauser's.
.