Mostly because I have basically all catalogs and have studied them extensively. It is mentioned as in the works once or twice iirc, but was never announced or in the stock listings = that's a sure sign. Questionable vaporware are usually announced or appear in a stock listing once or twice to then disappear, but those that never get that far have so far never appeared (any game, any platform).
The catalogs are a great source of info, although they are somewhat optimistic sometimes. There are some listings that so far seems to be incorrect, insofar as nobody has been able to find anything that proves them correct. North Atlantic '86 for Macintosh is a prime example, hehe. Currently there a couple of conversions that I'm very doubtful about (none for Atari though), but the rest has proven to be true, even some that I've clearly marked as errors in the past! So well, you never *really* know I guess..
The "In the works"-section is quite speculative though, there were lots of rumours started there, lots of planned conversions announced, and lots of them were cancelled. Unfortunately.. There were quite a few conversions canned that at least I would've loved to see.
