Hi fanshaw,
Fortunately our friend Noel has provided us with a solution, genial in its simplycity and simple in its geniality, which I call The Xmas Dry Film Test.
In this specific context, you load both suspicious cassettes with virgin outdated film you are ready to sacrifice, and start firing. You can fire the shutter without the back of the camera, or with the back and B speed, using the B time to mark each frame from the lens mount. When finishing the roll, the 36 marked frames will be your 'rentgan' proof of how the cassettes perform.
Cheers,
Ruben
PS,
The test was originally designed to know beforehand frame-space performance.