How to determine which old files are InDesign CS6 (32-bit)

Since InDesign CS6 is 32 bit and everything needs to be 64 bit for latest MacOS, how can I find out which old documents are still in CS6 and haven't been updated to a later version of InDesign? And once identified, do I need to open them in InDesign CC and convert/save them to latest version? I don't want to lose access to the old files once I upgrade and can't open CS6 files any more.