Most reputable military dealers will stand behind what they sell. It is suggested that if you do buy a piece and question the authenticity, talk to the person you bought it from before you discredit their reputation. Have more than one knowledgeable person look at it before you make a final decision. Second suggestion, and most important, educate yourself in your field. Quality reference books are your best source.
Reference books are expensive but worth their weight in gold to serious collectors.
Be prepared to spend some money. Good books are not cheap, treat them as part of your collection. There are also several good military price guides on the market and these are generally a lot cheaper than the reference books. There is a catch to the price guides. You need the reference books to know what it is you're looking for, unless you're lucky enough to stumble across a photo in the price guide.