Adam,
Judging from what people are saying, I don't think that you should think of this sword as a fake! I think that you should think of the sword as a "potentially" rare and expensive, and give it due respect accordingly. It could be 700 years old.
Pictures that are generally asked for, in order that an initial judgement can be made, are: Pictures of both sides of the overall blade, close up of the kissaki (point of the blade), close up of both sides of the nakago (tang of the blade) to show any possible mei (signature) and a close up of the width of the blade where the polished part of the blade meets the tang.
If the blade is at all rusty, don't attempt to clean it yourself and the tang is meant to be black/brown and never should be altered/cleaned/sanded etc as this indicates the true age of the sword, it is a natural patina.
Please do not do anything hasty, as an ill thought moment could cost you dear. Seek expert advice first, and quantify that advice on here first.
Best of luck!