I don't think anyone has answered Smoky's question since Moshe's post. How about helping the guy out?
One way to debunk the idea that the Holy Spirit is guiding the WT and the FDS is using their own literature. The April 15, 2010 Watchtower has an article about how Holy Spirit has been doing just that. It is easily disproved when you compare the current teaching on generation in that article with past teachings on the same topic, then ask: Was the Holy Spirit wrong in the past? Did it change its mind?
They will respond with "the light is getting brighter" crap, then ask them: Why would the Holy Spirit dispense incorrect light, or darkness, upon God's chosen people? Did Jehovah ever mislead the nation of Israel? Is the organization actually accusing God of misleading His people intentionally in the past just so He can become ever more truthful in the present and future? Does the Bible show any history of that sort of deceptive behavior from the Almighty?
This will only work if they're open to reason, which they probably aren't. They're brainwashed by a mind-control cult, don't forget. Google Steve Hassan and read, read, read.
Good luck, Smoky.