The easiest way to tell when you got a YJ is by the square headlights. if you see round lights and leaf springs, then it is a CJ. If it looks A LOT like a YJ, but has round lights and coils, it is a TJ. I take it you know what the JK (the new wrangler) looks like...
This is just the first things I look at to know what I am looking at... Obviesly there are lots of other things that are different, but that would take days to write. For that, like amuddyzj said, check wikipedia.
Felipe