Have you ever had medal ribbon arrive with a medal that you suspect to be replacement ribbon? If you are able to get hold of a loose thread there is a test to determine the material of the ribbon. This is a destructive test, so please do not start chopping up original ribbons to carry the test out, it will work just as well on a single strand of loose thread.
The theory is that by holding a match or lighter to the thread you can determine what it is made of.
Original ribbons, were traditionally made from silk. When submitted to a flame, the thread should 'fizzle', may curl up slightly, but will essentially just disappear. This video shows a loose thread of newly arrived Victory Medal.
As can be seen, the thread simply disappears.
Replacement medal ribbon tends to have a certain amount of plastic content and so when submitted to the burn test they will appear to curl up as the plastic melts. It will take a lot longer for the thread to be burnt away.
A useful test if you come across a very good quality reproduction ribbon.