I think there are several very obvious ways.
MIT Climate Scientist Richard Lindzen, in the Wall Street Journal, discusses one of the ways you can tell: by the ongoing pattern of intimidation of dissenters and stifling of debate associated with Global Warming in the scientific community.
If it wasn’t bunk, they wouldn’t have to punish dissent and censor debate, would they? If they weren’t liars and opportunists, they wouldn’t act with the ruthlessness and dishonesty which have become characteristic features of Global Warming orthodoxy enforcement.
A “for instance” seems obligatory, so I’ll just point to Scientific American‘s threatening to sue Björn Lomborg for daring to quote the special hatchet job they published on his book The Skeptical Environmentalist in a web-published reply to their attack.