As Paul mentions, the oil pan usually needs to be pulled for a more accurate identification.  Even pulling the heads and checking for a 3.800” bore isn’t absolute as a 292 block could have potentially been bored out to 3.800" and beyond.  If the bore checks out to be less than 3.800", then it's not a 312.  And to throw another curve out there, I’ve run across a 292 block with EBU caps that had its mains bored out to the 312 size so a 312 crank could be fitted without turning the 312 main journals down to the 292 size.

As far as identifying a 312 based on the large ‘dot’ on the rear crankshaft flange, there were quite a few 312 crankshafts that were cast as ‘blank’ or with no dot.  If the crankshaft has the 5/16” raised dot, it’s very likely a 312 but no dot being present could potentially go either way.