With all of the fakes out there, this can be a tough hobby to break into. Yes, purchase reference books and develop an eye for detail, for example, the spike on this helmet with it's little beads around its neck is simply awful (as is the shape of the dome, or body), but somebody who's never seen good references wouldn't know (just like the ugly Kurassier spikes on those Indian made helmets).
Get books, go to shows and museums, look on line in google pictures and start critiquing helmets there, and do what many here advocate: When you find a helmet you like, be very critical. Tony, for example, says (in his mind) "Prove to me you're original and what you say you are". You'll notice from Tony's web site (Kaiser's Bunker), that he's done very well that way.
Another thing to do is make a list of helmets you REALLY want, or foregoing that, just start collecting (which is what I did when I started)! This is one way to keep expenses down in this ever-more-expensive hobby.
You did the first step right, you asked for opinions and answers. Well done! If more people did that, we could put these forgers right out of business. So, welcome! Check out the forum and look at the helmets and comments here and enjoy the hobby... it's fascinating and enjoyable in many, many ways.

Ron