Mark, welcome to the forum!
Lots of clues, ensure that it
is all matching. Firing pin, breechblock, check the grips, etc. As a commerical it will or should be in the commerical, i.e. hidden manner... But 1920 Commercials also were reworks of old military from WW1.
So look for those clues too, i.e. serial numbering
not in the commerical manner, old proofs from imperial, etc.
To tell reblue;
Look for bangs, nicks, etc., that have been reblued over them.
Look for rounded edges, where it was reblued by bob the gunsmith and
not a actual arsenal.
Ed
PS: you'll hear this again and again, but books and studying is the most important thing. Buy Still's books, buy the era you are interested in and study the luger forums for all of the information.