Mark, I am also new to this, but from what I have gathered so far, it is all just someones opinion when it comes to nihonto.
Nobody alive today witnessed any signatures or saw any old blades constructed so, learned people voice an opinion of a blade and the "possibility" of who made it. Signatures seem to be worthless, and from what I have read so far, Japanese national treasures suspect. Of course I am very new and this is just my opinion, but I don't think there is any way to tell who made a blade so you will have to go by what you are willing to pay for any given nihonto.
Again just one new guy's opinion.
Kim Toth