There are a few ways that may help you decide if an item is genuine or not.
The first is to learn as much as you can about the militaria before making a purchase. Know how the items were manufactured, how to identify they are original, where and when they were used and how much to pay for them. Then ask the Seller the questions you have that satisfy your understanding that the item is genuine.
The second is to ask about the Seller's returns policy. The best way to stand by the authenticity of a military collectable is for a Seller to offer a money back guarantee on the sale. This is simply a return of the item after the sale if the Buyer is for any reason dissatisfied with it.
Sellers are able to offer this; on the listing it will say 'Seller accepts returns' and some instruction may also be provided in the item description to state under what conditions any returns policy applies. For example, a Seller may choose to offer a return within 14 days of the sale only. After that time, it is considered as sold, no return. Sellers may choose to only offer this at the postal expense of the Buyer.
If the Seller has not offered this and you want piece of mind, ask the Seller if this is available to you.
The third is to check out the reputation of the Seller by searching for them online. If they are new to Warstuff and don't have much feedback, check to see if they are an established militaria business with a good reputation. If they are, the chances are the item is exactly what they say it is.