[Thor]

The first rule I look at is:
1. If it is an expensive Luger, ya better know your stuff about that variation! BUY BOOKS, look at a lot of them. When a person is excited about a minty rare Luger and has the funds available to purchase it he BETTER know what he is buying. Expensive pieces or rare pieces are the most suspect IMO.
2. If they are weapons with the original finish that is now between 50-100 yrs old, the finish will most likely show some brown color under the bluing if exposed to a bright light. If you are going to a gun show take an extremely strong small flash light with you. Take the gun out in the sun if that is possible (if you purchase it)
3. Make sure the seller will take it back if you are not happy with it. Know what the return policy is in advance.
4. Look for wear on the high spots, side plate flat area, sides of muzzle, ends of rails.
5. Look for the appropriate mill markings and direction of factory polish. If you dont know this, learn it before you buy. Look at as many pieces of that variation as possible. This can take many years but will aid you in your over all collecting enjoyment.
6. Look for markings that look professional, stampings will normally show halos as the metal is harden next to the stampings, engravings, dont have this feature. Almost all factory markings are STAMPED.
If you are educated about the gun and the seller will take it back if you dont like it, then go for it, otherwise, put it off until you are ready.
Good luck!