Vulken fiber will usually be shiny if its been cared for properly and will look and feel just like plastic. Also if you rub a vulcan fiber helmet 3 times Leonard Nimoy will appear and grant you 3 collector wishes lol
Here is a detailed view on a rear visor of a vulcanfibre helmet with the typical artificial surface. Hope that will help you to identify these helmets.
In addition to what has been descrived above, one of the most obvious traits of a Vulkanfiber helmet is that the shell, seen from inside, is black, whereas it is usually brown (leather natural colour).