You just have to look at enough guns to start spotting the changes.
An ordinary reblue is hardly ever polished just the same as a new gun, it shows in the texture and color of the finish. The usual hardware store reblue will usually have been buffed hard enough to round off corners, ripple the flats, dish out screw holes, and blur markings. It may be applied over pits and scratches, and in the case of some military weapons, over marks that were originally applied after the finish.
A factory refinish of a gun recent enough that they haven't changed their personnel or chemistry can be hard to spot if the gun was in good condition except for the blue, but it can be seen.
There are true restorations that take expert examination, usually under magnification, to detect.
An ordnary reblue essentially wipes out collector interest, it converts a museum piece into a shooter. As one collector told me, 100% factory reblue is Zero ORIGINAL finish. Look up THAT in your Blue Book.
There is getting to be some acceptance of a full restoration with extreme care taken to preserve or return the contours, texture, and markings, but that is very expensive.
Bluing (or rebluing) consists of polishing the steel to the surface texture you want, it takes shiny steel to get a shiny blue, degreasing the metal, and boiling it in a solution of nitrate salts which oxidize the surface of the steel into black iron oxide, Fe3O4. Then rinse in hot water and dip in water displacing oil. It is really pretty fast, about 20 minutes, but takes a substantial setup of equipment and supplies, which is why it is not cheap even for an ordinary job. Nice jobs cost even more because of the skill required in the polish and preparation.
