As someone that reblues a lot of old airguns its difficult to tell for sure.

Some old rusty pitted boat anchor will need serious work to chemically or electrically remove the rust itself, the pitting of course remains and has to be removed mechanically with abrasives.

Removing that much metal to bring the surface below the pits is always visible even if you use all the tricks to keep the sharp edges and lettering as sharp as possible.

These guns are easy to spot but arguably have been saved from the grave for future generations.

Guns in good order with worn bluing through rubbing or localised damage from blood etc are a different matter, they can be stripped chemically and as the original polish is pretty much intact a very gentle 2000 grit polish will restore the surface to new with close to zero surface removal.

These guns are almost impossible to spot as long as the original manufacturer polish grade and colour are cloned, the usual giveaway is a 100 year old gun with a very high 20th century polish that's actually superior to that available at the time...