Antonio,
Mostly through experience. Rust or salt blue produces a semi matte finish, not shiny or polished.
Look for sharp edges and complete stampings. Any use of a buffer wheel unless used very expertly will always show signs of having worn surfaces and especially edges on a firearm. On the Luger pistol, a dead giveaway are the two scallops on either side of the chamber, which cut off the sharp edges of the upper receiver as it meets the barrel assembly. These cuts produced very shallow angle changes in the metal's surface which are easely buffed out prior to a re bluing job. Any shiny or gloss surface should be immediately suspect.
Also, no loss or minor loss of blue, (excluding the rare minty example) will generally indicate a re finish. Look also for a red underlying color to the bluing under very bright light, which is correct but does not show up as obviously in a reblue.
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