Is there anyway to tell if it was orginaly nickel platted or if it was added on?
If you bother reading about C96s, or any other similar semi pistols you will see that very few if any German army or even German commercial sidearms were ever nickel plated. A nickel finish is not necessary as a combat sidearm and actually serves to work against you IE increased visability. There is no reason for the Germans to have originally nickeled your mauser. In addition, its an added cost. I suppose some of the commercial examples could have been nickel, but I don't recall ever seeing a factory nickel example. The only times I remember seeing German military sidearms nickel plated were for presentation examples. If the gun is not marked in a special way, and has no engraving, it is not a presentation piece. Some ways to tell that your gun is not original nickel: plating over pits and other such markings, faint stampings, the gun being completely nickel (many times factory nickel guns have a few parts such as screws finished in another way) etc. The other good indicator is the history of the arm. If your gun is a military contract version (you never posted the SN or any markings, so we don't technically know) it is definitely not original nickel. You may hear stories about an Officer ordering the gun in nickel, or some other BS story, but rest assured, it was nickeled by a previous owner. Nickel plating GI bring backs was very common at one time.
As stated, you would have made out like a bandit to accept a $2000 offer when nice original ones don't often bring that much unless they have a matching stock or some other significance. If the gun and stock are mismatched, IMO the gun is worth less than $1000. When you say you saw blued examples bring $2000 or $2500, they were original collectible examples, and so they are different than your gun. Taking the $2000 would have allowed you to buy a better one for around the same money.