To the best of my knowledge you cannot visually tell the difference between an early and a late block by looking at the block heater area. The gusset across the front of the block is different, but this is of little use unless you are looking at a bare block. There are pictures in this article:
http://www.enginebuildermag.com/2006/07/rebuilding-the-international-6-97-3l-powerstroke-engine/
It really is best to check the block serial number. It is stamped on the flat space on the block next to the injection pump on the driver's side of the engine. It looks like this:
You must be registered for see images attach
...but hopefully with more favorable numbers! Check this chart:
You must be registered for see images attach
Hopefully that helps. The stamped SN is where the real info is.