The most bullet-proof 28 ga M-12 receiver discriminant is the cutoff raceway angle milled inside the frame. Parts can be stamped 28, receivers can be stamped 28. Faking the raceway, not so easy.

Another way to tell a "real" 28 from a converted 20/16 is cycling. Shell noses tend to hang up on the chamber opening of "real" ones. Conversion guns tend to work nicely.