I am currently trying to date some small Winchester components where the only change from old to new was the type of finish. 

Is there a non-destructive forensic method to tell the difference between the rust bluing method and Du-Lite blue?  On polished surfaces I can easily see the difference under a strong light but on matte finishes (such as those small parts Winchester tumbled in a Panghorn) I have a more difficult time.

Hoping there is something like a lighting frequency or chemical trick that can easily separate them.

Thanks!