Extremist traits.

Character Assassination.
Name Calling And Labeling.
Irresponsible Sweeping Generalizations.
Inadequate Proof For Assertions.
Advocacy Of Double Standards.
Extremists Tend To View Their Opponents And Critics As Essentially Evil.
Extremists Tend To Have A Manichean Worldview.
Extremists Very Often Advocate Some Degree Of Censorship And Repression Of Their Opponents And Critics.
Extremists Tend To Identify Themselves In Terms Of Who Their Enemies Are.
Extremists Are Given To Arguments By Intimidation.
Wide Use Of Slogans, Buzzwords And Thought-Stopping Cliches.
Doomsday Thinking.
Extremists Often Claim Some Kind Of Moral Or Other Superiority Over Others.
Extremists Tend To Believe That It's OK To Do Bad Things In The Service Of A "Good" Cause.
Extremists Tend To Place Great Value On Emotional Responses.
Some Extremists...claim some kind of supernatural, mystical or divinely-inspired rationale for their beliefs and actions.

This is actually part of an article by a journalist called Laird Wilcox from the 80s. It has become the staple for anti-extremism and people who generally want to stop hysteria and witch-hunts of some sorts.

This isn't the end of the story, we can see it happenning here. People can use the hunt for extremists itself as a witch hunt. Extremists can claim other people are extremists in an attempt to make it seem as though they cannot be extremists since they know a lot about the subject. I can think of dozens of elaborate plots used to circumnavigate the tell-tale signs of extremism.

Generally if someone doesn't have facts to back up their argument or refuses to discuss an issue relative to how much they seem to care about the issue, they are an extremist.