I have found a stereo microscope is the ultimate tool for diagnosing re-blue. Typically local jewelers will let you use theirs to inspect an item although of course it helps if they already know you.
I have a stereo microscope, and I can tell you that all Lugers, even high-condition ones, have rust (pitting). On a high condition gun the pits are not visible without magnification but they are there. It is a question of degree. If it's a re-blue you can see rust in the base of the pits.
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