If you have military museums in your area, it's a good idea to stop in and see what the originals look like. Check antique stores and get a feel for the look and smell of an original as well.
Some of the fakes are really quite deceiving, so experience, research and a discerning eye are important. I think it was Tony who said he looks at a helmet and says, "Prove that you're original." This is a good idea and the best idea is not to get carried away at a sale or auction and pay a lot for something that is really not what you're looking for.
Other suggestions:
A list of what you want to buy and what you want to avoid
Start a reference library
Make notes of what you see on helmet (Markings, cockades, trim / furniture types / colors, etc) and check them against your library sources. Sometimes, the source book can be wrong, so HERE on the forum is the best place to ask questions. There are some real experts here.