For the main SkyUI quest, there is a script file (that is NOT visible and it should NOT be visible) called ski_main.
Inside this script there is a property called "ReqSWFVersion", that actually is a string with the version number of SkyUI. (Tested in version 4.0, 3.1, 5.0, and 5.1)
Now, you cannot really get this property with ease.
First because you DON"T WANT to put a hard reference to the SkyUI quest (DON"T DO THAT it is just a BAD idea.)
But you can get the actual script instance using the SKSE functions.
And you can access this property. Without put a bad reference to SkyUI.
The problem is: how can I compile now my script?
PapyrusCompiler is telling me that "ski_main" is unknown!!!!
This is because this script is not, and should be NOT, provided.
But you can create a fake script (only the .psc, then .pex is NOT required) that has the definition of the property (nothing else is required.)
scriptName SKI_Main extends SKI_QuestBase
String property ReqSWFVersion Auto