Reading, looking over other guns, asking opinions...
Go to several of the websites listed and check prices and condition.
http://www.lugerforum.com/lugerlink.html
Ensure it is, as stated.
If all matching, it needs to be.
Ask for a 3-day look at it and return if not satisfied. Most dealers will allow that if dealing off the internet or phone.
Also, it is much cheaper in the long run to buy a $600 worth of books, than one "shooter" luger.
Ed