Original finish was either nickel or blue. You might find a stamp under the grips or somewhere else that signifies the original finish, B or N. Original finish on nickel guns should include case hardened trigger and hammer and blued trigger guard. If your gun is nickel, look carefully at the finish and if there are any rounded edges, refinished trigger, hammer, trigger guard, pits with finish over them, general poor preparation under the finish, wavy metal in flat areas, etc. If everything is clean and crisp, you probably have an original finish gun.
Since there were over half-million of these made, there are still many around and value is just not very high, except for exceptional specimens, with near 100% original finish.