Sketchup.version and Sketchup.version_number
Returns the current version number.
The Current Version of Google SketchUp Pro is:
6.0.515 for Windows XP, 2000 and Vista
6.0.514 for Mac OS X 10.4 or later
BUT it's also that for the Free version ?
However, since the Free version lacks some features in the Pro one then you can look for their absence ?
For example:
is_pro=Sketchup.find_support_file("Skp2Acad.dll","Exporters")
is_pro is true for Pro and false (nil) for Free...
simple but works