I hope negationist is the good term. What I mean is how do you refute or convince someone who thinks something in history did not happen?
Take this post for example: http://www.reddit.com/r/AskReddit/comments/2pw2fq/serious_holocaust_deniers_how_do_you_refute_video/
Did any of you tried to answer these people?
What convinces me the most the methodology with which you work. I have followed two history courses at university level (Introduction to Western civilization and another one on human rights) and my teacher explained the way historians work and the academic way seems to be the most strongest way to develop a good reasoning (whether it's in History or in other fields). But I hardly see myself explaining that to someone from this thread.
Do all historians agree that WW I & II happened?
Show them primary sources to show them what happened. Allow them to show their sources and work to refute them if I can. If they do not believe that then nothing will change their mind and I won't waste my time.
Personally, I like using something I called the "Titanic method". Pretty much everyone knows about it--it's a big ship that sank in 1912. But until relatively recently, there was very little physical evidence of it--we relied on the accounts of thousands of witnesses to form a narrative. It was only when the wreck was discovered that certain elements could be proved/disproved.
For instance, many of those present during the sinking stated that the ship sunk in one piece, and simply glided underwater. Others claimed the ship broke in two first, with the stern floating for a few minutes after the bow broke off. Until the wreck was discovered, the first view was the dominant one, when physical evidence proved that they were wrong.
Similarly, witness accounts conflicted in other ways. Were gunshots fired as lifeboats were loaded? How many boats were in the area of the Titanic? Etc, etc, etc.
With all these conflicting testimonies, can we really believe the boat even sunk? Well, yes, of course we can, because nothing else reasonable accounts for hundreds of people in lifeboats in the north atlantic, and now we have a shipwreck to confirm it. And of course, some of the witness accounts were accurate.
A popular Holocaust denier technique, which you can see in the top undeleted comment in the thread you linked, is to point out that since some stories that circulated were proven false (making human soap, shrunken heads, etc) all the accounts of the Holocaust are problematic. And it's true--witness accounts can be dodgy. But that doesn't mean that it's not idiotic to simply chuck all of them in the bin. Unless, of course, you're willing to buy in to my theory that the Titanic never sunk, since the eyewitness evidence of it is so unreliable.
Depends on the pretext for their opinion. If it's a belief taught to them from an authority then the rational way of explaining a conclusion through a well thought out and supported premise is the way to go (or make a sad movie).
If the pretext for their opinion is political gain then the only way to stop that is to change their political context to make such a belief a loss rather than a gain.