For me I really just take each stain at face value. If you collect vintage long enough you will encounter all different types of stains/dirt/grime etc.....
Most of the times I have had good luck getting paper remnants off of pre-war cards. I have erased some things that worked well and others (oil based pencil or pen) don't come off well at all. But to answer the question it is really just from experience in knowing what the extra residue is from. I have used magnification to see if something was a remnant or paper loss.
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Leon Luckey
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