The way I believe it works makes me think that

Using FormatA.potx to create SalesJan.pptx (example) then the built in Template property for SalesJan.pptx = "FormatA".

Using FormatB.potx to create CostsJan.pptx (example) then the built in Template property for CostsJan.pptx= "FormatB".

I'd think that would be enough


If you're concerned, then when a new PPTX is created, you could add a CustomDocumentProperty ("OriginalTemplate") = BuildinDocumentProperty ("Template") and test against that

Personally I think that's overkill, but then you should never trust Users