It takes experience and just paying attention to the forums and reviews to figure it out. In today's instantaneous web world, if a company is releasing an all-new tooled model of a subject, then it is preceded by a lot of speculation, online buzz, pictures of test shots, box art photos and the ever present experts critiquing it based on test shots. If the kit being released has already been produced but outdated, you will see eBay and the online trade forums fill up with the older kit being dumped in anticipation of the new release.
Very rarely is a totally new kit just sprung out upon the market. It doesn't make sense not to create a stir in the market and make the kit a hot item before it even hits the shelves.
Certain companies almost always have someone else's kits in their box. Companies like Italeri, Revell of German, Hobbycraft, Zvezda and many Eastern European manufacturers always reissue other manufacturer's kits. Airfix too, at least in 1/35 scale, they do not have their own line and are almost always someone else's kit.
So when a company like Revell of Germany announces the 1/35 scale M3A1 White Scout Car, you know without even opening the box that it is the old Peerless Max kit.
Why? Well, if it was a new-tooled kit, some modeler expert that is near an actual White Scout Car would have told a friend that he was working on a new kit. That friend then drops hints on the various websites he frequents that there is a new scout car coming. Then the assembled experts start speculating on whether the kit is a new tool or reissue. Someone at a major show/event reports they saw the test shots and that it is not the old Peerless kit, and so on.
It's real hard to surprise us with a new kit in today's global world.
For the record on the M3A1 White Scout Car, it was first released by Peerless Max (#3507) in the early 70s. Airfix then released it around 1976 followed by Italeri. During this time it was retooled to delete the canvas cover and the crew figures were removed as well.
Testors then released it in the yellow box in the 80s. Italeri reissued it in the 50th Anniversary of D-Day box in 1994. Italeri reissued it again before the molds were passed on to Zvezda. They released it without the cover and then retooled a cover and released it again.
Currently, Revell is releasing the latest version with the cover. I've still got an original Peerless version and the 50th anniversary version.