I'm sry, following that logic, we can't say if an item with low merch value is prenerf or isn't.
Ofc we can say one with high value Has been dropped, but the contrary is fals.
So why would you consider a q8 sword with low merch value less valuable than a high value one, even if the low one was dropped before (with no solid proof, only the owner's word) ?
Imo, considering items prenerf according to their value isn't stupid, but saying they are more valuable than the ones still droppable today is stupid. Since you can't tell when an item droppable nowadays is old or not (know his date of loot*)
EDIT:
Btw ikki, just to answer your question, the easiest way :
Celestial sigils are an example of prenerf dif merch value 
I know it's not really a "drop", but still