The main visible external difference between a 427 and the others in the FE family is the 3 evenly spaced 9/16" headed bolts on each side of the block. These bolts are for the cross bolted main caps used on the 427. This is not always a foolproof method of ID though because it is possible to install the cross bolted maincaps on any FE block. I have even heard of dishonest types simply installing the bolts so the block looks like a 427. 427 side oilers have a raised rib on the left (drivers) side of the block with some fittings threaded into it. The rib is the side oiler
oil gallery. Most 427 side oilers also had screw in freeze plugs but once again, this can be done to any FE block, although it's not that easy. The only proof positive method of externally IDing a 427 center oiler block is to verify the casting numbers.
HTH,
--Mike