For a good overview of how to spot a global warming denialist, and denialists in general, check out this article by Mark Hoofnagle, which lists five routine tactics denialists use:
1) Invention of a conspiracy to suppress the truth
2) Cherry-picking of data and quotes
3) Fake experts
4) Moving the Goalposts
5) Logical Fallacies
I'd add a 6th tactic as well: avoiding the scientific peer-reviewed literature and publishing popular books instead. It is important to be able to distinguish between someone approaching an issue from a good-faith scientific contrarian position and a denialist. Science thrives on debate and disagreement. However, scientific dissenters don't use the tactics above. Einstein, who denialists like to bring up as an example of someone from outside the scientific mainstream being vindicated, didn't resort to any of that. He published his findings, and was vindicated by experimental data. Denialists don't do this, for the simple reason that they rarely have anything to publish. That should speak louder than it does.
I encourage reading the comments to that thread as well, as the denialists show up in droves to provide real life illustrations.