According to http://en.wikipedia.org/wiki/Imax#Theatre
(though I can't find it on IMAX's website that they reference), imax theatres are classified as "classic design" or "multiplex conversion".
Films must be modified to be displayed on the classic design as the screen (like the film) is 4:3 format. I, personally, object to films being modified from their intended presentations. The other option may be to not use the entire projection screen, which is probably OK for me but also probably not for most people. Further, IMAX claims that the systems meet the same technical specs regardless of theatre type.
I'd suggest, therefore, that what constitutes "real IMAX" is a bit of a grey area. If one absolutely had to categorize (which actually I think is a pretty decent idea because I've been to some IMAX presentations that weren't as good as my moderate-quality home theatre), I posit that what is really needed is a catalog of IMAX theatres based on those three categories. Within the two non-dome categories also should have the screen size noted since this is actually the main difference (assuming that theatre owners are doing everything properly per specifications etc from IMAX). This way, you know if you are going to a "classic" theatre where they may crop your film (this is how I saw Star Wars 2 - Electric Boogaloo), and viewers can make their own decisions as to visual impact based on experience, and decide their own screen size threshole to pay the IMAX premium.
Of course this is probably what viewers already do... go to an IMAX presentation at a given theatre and decide if they liked it enough to go back. Of the IMAX theatres I've been to in the last year or so, one had the sound so high I couldn't enjoy ANY film in there anyway. We left and received a refund. One, however, has the sound at a reasonable level and a large enough screen to satisfy me and make it worth the cash.
The truth is that, almost inevitably, if I don't care about seeing some specific film in 3D I will almost always wait until I can rent it on gru-ray and watch it in my own home theatre. Even if it's due to the fact that I can eat BACON while I watch a film.