5.9 worth looking at ?????????? from local craigslist

I've got a 5.9 that the seller told me it was out of a '98 truck. How can I tell for sure what year this motor is? I've been searching for days on the Internet trying to decode the serial number on the block and can't find anything. I have discovered that there are different spark plug part numbers and there gapped different depending on the year also.