Here is another subjective method that I use - BUT you must have good vision. You can hold the polished surface up and look at it at a very low angle, almost like you are looking at the edge. Have a low light source behind it (I like to look at reflections from outside light through a window). You don't want bright light. I can see the imperfections (scratches, flat/unflatness) pretty well, but I do have really good eyes. Not for everyone, but something you could try.
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