So, without a point for comparison or any way to distinguish the original, how do I go-about buying inserts?
This is where I used to point people in the direction of the MPT archives that have fallen into the dustbin of history. There should be some info in the multiple NSFGE archives (search 'minty white') if you can stand the smell.
You can always pick up a real insert for a lesser title on the cheap for some basic comparison, but if you are looking at a super high demand title then you should look for the specific information.