Hairy,
If you are speaking strictly of frames "proof marks" are of little use as they were stamped primarily on receivers, barrels, and breechblocks.
An Erfurt frame will have an Imperial inspection mark, a crown-over-letter, stamped on the front of the trigger guard, and very likely (but not always) a crown-over-RC stamp somewhere else on the frame. (C/letter marks are also found on most other parts of the other parts of an Erfurt-manufacturerd Luger.)
Simson and Krieghoff Lugers will be found stamped similarly to Erfurts, Simson usually with an eagle/6 and Krieghoff with an eagle/2.
Regrettably there is no corresponding method of differentiating DWM frames from Mauser frames, as neither manufacturer stamped identifying marks on their frames. If you can directly compare the two you will find that the font style for the frame serial numbers are different, and likely struck to a different depth, but this is not a reliable identifier while observing any given frame.
If you are examining a frame complete with its small parts, and are certain that it has not been refinished, you will find that DWM frames, without exception, are rust blued with polished-white interior surfaces and strawed trigger, takedown lever, and magazine release. Mausers from mid-1938 on are salt blued, with blued interior surfaces and blued trigger, takedown lever, and magazine release. "Original finish" is the key here, and it is no help with Mauser Lugers 1934-1938.
If you are simply examining an unmarked Luger frame I'm afraid that, as drbuster pointed out, Gibson's forensic descriptions are your only recourse.
--Dwight
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