Ah, but there ARE markings on the receiver that tell you what 'size' it was when it left the factory. The serial number is the key here- serial numbers ending in an M indicate the gun is a 12 ga. full size magnum frame set up to handle 3" shells. Serial numbers ending in V however were full size 12 ga. frames originally set up to handle 2 3/4" shells.
As with so many things in life, you just have to know the code... 8^).
Note that the only difference in receivers as far as "normal" (i.e., not 3.5" super maggie numb) receivers in 12 ga. 870s is the ejector. Non- magnum receivers can be altered to handle 3" shells by changing the ejector but this will require refinishing the receiver and is not likely worth it just to be able to use 3" shells- you should be able to accomplish 99% of your shotgunning quite successfully with 2 3/4" shells.
hth,
lpl/nc