for a sword to be regarded as a "true" Nihonto it should be made using tamahagane which is steel refined from sand iron in a traditional way. It should then be constructed by welding steel of diffent carbon content through a process of heating and hammering numerous small pieces together. This is then folded and drawn out in to a sword. It is then differentially hardened in water. This methodolgy has been used for over 1000 years so the material and methodology are the key factors not necessarily when it was made.
Once a sword is made it is not possible other than through destructive testing (even if then) to determine whether a blade was made from tamahagane. It is possible to see if it was hand forged and folded and whether it was differentially hardened in water.
As Dave mentioned there were numerous methods employed to make swords for the second war. Some employed using traditional methods but imported steel, some partial hand finishing and then ffinally quenching the blade in oil rather than water. This was much more forgiving and didnt produce the high rejection rate water quenching did.
Determining how your sword was made will be a worthwhile learning curve for you. As said before it is most likely not traditionally made but you can still see if it has been folded and try and determine how the hamon was created either with oil or water quench.
Edit Hell I am getting slow! sorry Joe I am repeating your post due to incompetent typing.