Most have a Read Me file in the folder for that scenery which lists the versions and changes...
Most have this, because the concept of a "version" with a traditional scenery that resides just in the scenery folder, it's easier to define.
Our sceneries are spread in different places, some of their data don't even come with the scenery, for example docking systems are only coming with GSX now. In addition to that, we are able to fix lots of smaller bugs by real-time patching of the scenery, by simply upgrading the Python code that runs some of it, and that is automatically updated with a Live Udpate, even a minor, so you don't have to download anything on your own.
Without even mentioning that, most of the time, lot of bug fixes in a scenery are related to the AFCAD, and we can automatically update THAT one too, either when you run a scenery installer you might already have (even if it's old), or when you run a Stand-Alone Addon Manager installer, or the GSX installer.
Nowadays, the most relevant reason why you update scenery installers, is to keep up with new version of the sim ( FSX doesn't change, but P3D had lots of releases ), or to add more and more features to the installer itself, usually to defend against other bugged 3rd party installers, or trying to catch more and more things that are wrong on the users systems, like corrupted files and such.
This results in updating the version in the installer, in case the user would report a problem related with the installation, so we can check which installer he used, but it's very likely the scenery files inside haven't changed, and what HAS changed, would have been grabbed automatically by an old installer just the same, because we TRY to fix things by Live Update, if possible.
That's why, unless you are running a VERY old version that came before a major update to a scenery, for example O'Hare before 2.x, or Zurich before 2.5, in that case just download it again, it's usually pointless worrying about installed scenery versions, because you don't usually need to.
We also have a "scenery outdated" notification system. Be sure that, if we'll ever publish an update that cannot be sorted out with automatic downloads during the installation or the Live Update when FSX is running, we would use that system to prompt you to redownload the scenery installer.
