IIRC Colt New Service were offered in 5-1/4" and 7-1/2". While I have not been fooling around with Colt New Service revolvers for about 25 years, I had two in 45 Colt, a 5-1/2" that had been rechambered from 455 Eley, and a 7-1/2" that had been rechambered from 38-40 (both before they came to me), and I worked on many of them. Never say anything shorter than 5-1/2" factory barrel, although I saw many of them that had been shortened for carry. The easy way to tell if the barrel has been shortened is the lettering on the barrel, it is pretty much centered lengthwise on the barrel, and the Colt DA 45 marking is towards the frame on the left side.
The best way to tell the old nickel is that it is kind of yellowish, more modern nickel is brighter with no yellowing. I have never seen a Colt New Service factory nickeled, but it would not surprise me if they made them.