We understand that if a user has the most recent version of a GuiXT script on his desktop the application can identify this and not reload the script. Assuming that we are using the SAP Web repository / RFC method for accessing scripts, how is this check made by GuiXT to validate that the local PC's GuiXT script is the most recent version?

To help you understand how scripts are downloaded, here is how GuiXT caching mechanism works:
GuiXT maintains a cache directory to store all the scripts. This directory can be specified in your profile (Directory0 in guixt.ini file). If replication is turned on, GuiXT will download/check all the files once per session. Next time around in the same session, the files are not downloaded but simply used from cache.

There is a version number mechanism which can further reduce the number of RFC calls. In your session script (esession.txt) put a command called versionnumber. GuiXT will now maintain a version specific cache. In this way if the script is found in the cache, it is never downloaded (or checked for differences). If you change a script on the server, simply change the version number and all the scripts will be downloaded again.
See the "VersionNumber" command for more details.