If X sues everyone who calls it a cult, you probably have fairly strong evidence that it is a cult. I think you have to evaluate how reasonable the lawsuits seem: Would this be the sort of thing an established, legitimate company you trust would sue over? Ciphergoth's example of Alcor suing people who accused it of fraud, for instance, strikes me as potentially reasonable - I think most companies would take legal action there.

As a general heuristic, I'm not aware of any respectable business which is often referred to as a cult and takes legal action against this. Apple does not sue people who talk about their fans as zealots / blind / cultists / etc., for instance. It's also, as far as I know, not a solid legal case, unlike fraud, intellectual property, etc..

Thus, I'd generally conclude that "X sues anyone for calling them a cult" is a pretty good heuristic for "X is not an organization I trust."

Whether they're actually a cult seems moot, past that :)