It's good to test a swatch, and you can always burn-test to to be more certain of fibre content.
But if it was my fabric and it didn't seem off, I would probably just assume that it's 100% polyester and that there's very little I could do short of throwing the entire thing in a fire that would damage it, haha. :P
If it doesn't seem like the common 100% polyester type, then testing is about all you can do to see how it will go.
Actually in the pipes:
- Remaking Ser Loras doublet.
- Luke Skywalker, A New Hope
- Wind Waker starter costume in swim material
Eventually: Paul from Dune, Faramir, Harry Sullivan