There is a difference in our current society on how Right and Left extremism is perceived.

...you're certain that the opposing extremists are making gains at your side's expense.

Even accepting the canard that National Socialism is somehow "extreme Right,"

...you have a hair trigger for perceived insults, even when none is present (I never mentioned the Nazis).

If you think collectivism can't eventually take total control of even a free nation, you're naive.

...you constantly fret that the other side will win, with horrifying results; while never imagining that your own side can win, and never see how that might be just as horrifying an outcome to other people.

We pay taxes without complaint that European serfs would not have tolerated

...you can't stay on-topic, but inevitably bring the discussion around to one of a handful of your hot-button issues.

BTW: Serfs couldn't vote and couldn't give their assent to taxes; Americans vote and have given their assent to our tax system through their elected representatives. At this point you'll quote Ayn Rand on "consent of the victims", which leads to another criteria:

...you're sure that anyone who exercizes his/her political rights in a way you disagree with must be mentally deficient and shouldn't be allowed to vote for fear he/she will vote the "wrong" way.

No doubt there are many more ways to recognize an extermist, but these will do for now.

--Robert