How can I tell if an honor society is legitimate?
In all of my experience: no one within academia cares about honor societies whatsoever.  To expand on that: in academia we routinely (closer to constantly) evaluate people based on their academic qualifications, in a very serious way: i.e., the "winners" get offered substantial scholarships and/or jobs.  Not once have I ever heard anyone mention membership in an honor society in any of these decisions.  Not for faculty hiring, not for graduate admissions, not for picking TA's...never.

When I was an undergraduate I did join Phi Beta Kappa (junior year: whoop de frickin' doo) and Sigma Xi.  The way I recall it, I was moderately encouraged to do so by my undergraduate program.  And I must say that the amount of money that I gave to these people -- if any; maybe my school paid it themselves? -- was truly negligible: at most $40, or something like that.  Based on this money, for many years afterwards I got:


Regular newsletters from the organizations that made what looked like reasonable attempts at giving me some kind of scientific / academic contacts.  (Not that I read them...)
Semi-regular very strange calls offering to sell me gold-laminated books and other such frippery.  Often these were done with enough of a "hard-sell" solicitation to get me a little steamed: one caller spoke breathlessly about the gold-laminated books and at the end asked me how many additional copies I wanted to order at a discounted price.  I mentioned that she skipped an important step, and she asked what that was.  I revealed that at no point in the conversation had she asked me whether I was even the slightest bit interested in buying anything from her.  The rest of the conversation was brief but tense.


Based on these experiences, whenever any student asks me whether they should join an honor society -- the ones which are not supposed to be scams, like Phi Beta Kappa -- I say "I do not advise you to do it if you have to pay more than the most negligible amount of money."

However, I have occasionally talked to other faculty and university personnel about this issue, and their experiences are not always as negative as mine.  What I suspect may be true (but I have unfortunately never had the occasion to find out!) is that: academic honors societies may be useful for people who are leaving academia, by providing some kind of modest continuing contact with the academic world.  I mean, the idea that I need to open up a bi-annual newsletter and read about what other academics are doing makes no sense for me: I learn about what other academics are doing by going to work.  But for people who go off into industry -- or something else -- maybe it is valuable.  On the other hand, I have to imagine that, as with so many things, the internet makes this kind of thing largely obsolete: the amount of access that any citizen of the world with an internet connection has to current academic activity is greater now than what any professor had twenty years ago. 

One final question: does anyone care if your CV says you are a Phi Beta Kappa (junior year!)?  I suspect not -- as someone else mentioned, these awards are given for your GPA, and you put your GPA on your CV anyway.  But I would, as ever, be interested to hear other perspectives.