He had ultimate-insider's access to top secrets of the Soviet Block. After his defection he was thoroughly debriefed by Western spy agencies. He has a few books out.
I admit its practically impossible to verify his writings unless one has access to the KGB's and satellites' archives. But the puzzle pieces he contributes fit so nicely with lots of smaller pieces already in the puzzle of what we know about modus operandi of the Soviet/KGB propaganda machine. There is a chance that he "INVENTS" all his stuff, but I don't think so. Nothing he wrote in all his articles, and I try not to miss them for the last few years, struck me as unbelievable.
It's more like Hess, Rosenbergs and all McCarthy "witch hunt": nasty individual he was or not, the funny thing that witches WERE there.