This is probably going to require actually going around to each PC and checking by hand, to some extent. You can do a pretty good job with the User State Migration Tool, but because users generally can save documents in other places, it's a pretty safe bet that at least one of them will.
So, how I've approached this in the past is to take an image of the old hard drive with a tool I like, perform the upgrade and user state migration, and then tell the users to let me know if they're missing anything. While they're getting acclimated with their new OS, I'll look through the images I took and see if anyone's got a cache of files saved outside of the My Documents folder, and copy it over to their new computer/OS if I find it.
You can also even just dump the whole folder structure of the old PC onto the new one, in a folder named
Old Computer (or similar). Some users really like that, some don't.
Then put the images on an external hard drive or USB stick, and store them in a drawer somewhere, because it's also a safe bet that sometime a couple years from now, someone's going to complain about not being able to find a file they're sure they had before they got a new PC, at which point you can pull out the drive with the images on them, dust it off, and present them with the image to search through.