You've somewhat answered at least one of your own questions there, it is safest to "just get the lot and do them all". The installer programs on these .dmg files are now 'wise' enough not to destroy or duplicate anything you already got, I might add, so that shouldn't be a concern. If, however, something has been updated within the things you already got, grabbing everything will ensure you get that.
But if you don't fancy the idea of waiting for download of something which you highly likely already have (or, like me, have "foreign dl cost" to consider), you can indeed find out what you got in several ways. The simplest is actually from within the program itself:
Under the Help menu there's a very handy "What add-ons have I got?" which tells you precisely that, numbers & all. You can easily copy that text and do a comparison at your leisure.
A slower method is to look into your RT Demos folder and make notes of the set number of the folders you find there, and an even slower method (as a last resort) is to look at the list of all Realtracks on pgmusic.com main site (here) and do a comparison. Neither of these two options is likely to save you any time, though.
So again, my advice echos your own words: Grab everything and run every installer! - Oh, and welcome to the 'club' cool

Just because you can, doesn't mean you should!
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BBox 2022 Audiophile, Mac Pro Intel, OSX 10.6.8, 800x600 (TV VGA)