Take the grips off and look for a N stamped on the left side of the frame near the bottom. Some times there is a N stamped on the rear face of the cylinder.
I think that means re-nickel. If there isn't a N on the gun, it was probably blue originally. At least that's my understanding from dealings on a different gun.
I'd defer questions like this to experts like Doc44 for an answer. I've never actually seen a Mod 25-2 shipped from the factory with a nickle finish although I've heard reports a few exist... Good question.....