Bill, I would say that it is really a matter of the level of your experience in examining similar guns...
In my lifetime, I have examined hundreds of Lugers, and in the last five years I have had the privilege of examining literally thousands of highly detailed photographs of Lugers, so I am somewhat comfortable with knowing what I expect to see when I look at the "finish" on a particular model of a Luger... that is reinforced with having read virtually all of the published Luger references, and I still own many of them.
Crispness of the edges and markings can be a good indicator, but advanced collectors (of which I am not... I collect information since I couldn't possible afford all the Lugers I want) have some pet tests that they use to determine authenticity of finishes. One of these tests, if my memory serves me correctly, is conducted with Q-tips and a compound known as semichrome (SP?) which reveals the level of oxidation on the surface. One of the collectors (like Tom Armstrong) will hopefully chime in here and make a more detailed report.
If a gun has been "reblued" it is seldom that some metal finishing has not also taken place... A Luger that is shinier that similar contemporary models that left the factory at the same time is likely to have undergone post-factory buffing before being refinished.
Prior to the end of 1937, virtually all Lugers were rust-blued and only externally. The interiors of these Lugers (Like the Magazine well and moving part surfaces should be found "in the white" when the gun is disassembled.
An original "plum" colored extractor or ejector would "suggest" that the gun has not been refinished, but there is no guarantee that these parts if in good condition, were not removed before the refinish process just to make it look more authentic when completed.
"Thor" has said that the muzzle of most Lugers is not blued... and my experience supports his theory, but I can't document this condition to be true for all Lugers.
The Plum color seen on many guns, including Lugers is a result of incorrect temperatures being used during the bluing process or maybe even some contamination of the bluing solution.
Chipped grips, are no indication of original finish in my opinion, because it happens so easily during cleaning. It is just proof that the gun has been disassembled at some time and the person removing the grips probably used an expletive adjective when it happened.
Rounding of corners during the buffing process is an easy thing to do if you are not careful and have a light touch, and the proper holding tools... "Thor" is a master at metal finish restoration and hopefully he will also comment on this thread.
I hope this has been helpful.
Bill, your profile indicates that you are a new forum member (registered this year) but I seem to recall that you used to post on the "old" format of the Lugerforum. Am I correct? or has my ancient brain made a mistake?