[wwbirds]

The old way to measure choke in fixed 12 gauge is with a Canadian dime. If the dime will not go completely into the barrel it is full choke. Not the most accurate but if you just want to know if it has been modified that is the fastest way.
More accurately if you have digital calipers measure the choke and compare it to published Beretta info online. Personally I would not shoot 3 inch mag steel in anything tighter than improved cylinder not only for safety of bridging larger shot sizes but also because every 3 inch steel load I have patterned shoots better through improved cylinder.

Should you wish to have the fixed choke reamed out to IC many gunsmiths will do it for $75. Had a 10 gauge done here several years ago and just acquired another one that I plan on having opened up before goose season.
Rob