I just look at my bookshelves. I know pretty much what I have (and I have quite a lot).
The one exception is MERP, which had a lot of supplements for the same region or that were pseudo-replaced with bigger second edition books. There are also some that I've read but don't own in paper form. So for MERP I do have a paper 'wants' list that I take whenever I go somewhere that might have any, to stop me getting any duplicates (or to stop me getting confused and not buying something that I actually don't have)
The one exception is MERP, which had a lot of supplements for the same region or that were pseudo-replaced with bigger second edition books. There are also some that I've read but don't own in paper form. So for MERP I do have a paper 'wants' list that I take whenever I go somewhere that might have any, to stop me getting any duplicates (or to stop me getting confused and not buying something that I actually don't have)