Bluing is 1 key, but look at the flat spring on the left side of the bolt. It should be a purplish color (nitre blued) and usually the safety will be a straw color (lower temp of nitre bluing). Often times, the lugers that come in have been reblued improperly, thus destroying value. Lugers will have a date code stamped on them. Pre WWII has higher value. Several companies made the Luger. Deutsche Waffen were the highest vale. Walther, Krieghoff and several other companies took production over, I believe, after WWII. Could be wrong on date though. Pre WWII in excellent condition can go as high as $3200 that I've seen.