I could make a bunch of stuff up, but that probably wouldn't be any real help.  I'm not really qualified to talk about it intelligently, I just know that the early engines had an aluminum spacer plate and there were issues with it.  We have two of them, but they both had had heads replaced before we got them and I was advised that they probably wouldn't be any further problem.  Same applies to the injectors....I know there were problems but I don't really know what they were, and I was told that ours were probably fairly safe. 

If I was looking at a truck with a 2YG or 7LG in an auction type situation where I had to make a decision without any qualified advise, I would probably  base my guess on whether or not the head appeared to be original....head looks like it hasn't been off, I'd discount heavily.  Head is unpainted and appears to have been replaced, I would discount less.  Then I'd start at about where I thought it would part out and work from there.  It's not foolproof but it would increase my odds. 

If it is a 9CK engine, these issues would not apply.