One of the biggest was there were animated gifs at the time showing that the space each line took up across the page was identical to the document created in word.
The evidence was legion.
In fact, Dan lost all credibility on the Friday following the 60 minutes broadcast. He actually had the audacity to say they were most likely genuine and even flashed, for just a second, a document from the 70’s that had superscript. Before the internet, that would have been QED. But we were able to vet everything and expose him for the bald face liar that he was.
To this day I call Dan Rather the first well known casualty of the free flow of information afforded by the web.
Here’s the timeline: http://mediamythbusters.com/index.php?title=Rathergate
Dan must not think anyone watches him. I caught a part of that broadcast in which he had his source on to interview. Dan asked the guy about him vetting his sources and information to which the guy basically said that he hadn't done any confirmation of information and that he had expected CBS to do it. The look on old Dan's face was priceless.
Not to mention that the source had been presented as "unimpeachable" the week before and had been kept secret presumably to keep him from being killed by the Bush cabal. Then a week later brought him out. Either keeping him anonymous was done just because he was an unreliable source or Dan "burned" a source by endangering his life. Either way it's something an honest journalist doesn't do.