There may be a more elegant way, but this method works for all versions of Word, Excel, and PowerPoint 2007 or later on all operating systems:
- Change the file extension from .pptx to .zip (in truth, that's all the file really is, a compressed zip folder)
- Open the zip file and then the docProps folder
- Open the app.xml file in a text editor (e.g. TextEdit)
Look at the last tag, <AppVersion>. The number in that tag will correspond to the internal version number of that version of Office, which can be found in the Title/Version column on this Wikipedia article. For instance, one document I tried this on lists the app version as 14.0000, which corresponds to Office 2010 (Windows) and Office 2011 (Mac). As far as I can tell, there is no way to tell if it was made with the Mac or Windows version because the files created by each are functionally identical.