I occasionally pick up fabrics from thrift stores, and I can usually tell the difference between cotton and polyester, but there are times when I'm stumped by the blends. The ladies at So Sew Easy put together a good video on how to determine if your fabric is synthetic or the real deal. It's important to know what you're working with before it's too late. Case in point, I once made a whole wack of HSTs and when I went to press my first batch, one triangle on every HST shrunk because I didn't realize it was polyester/cotton. Why did the triangles shrink? The poly fibres in the fabric melted from the hot iron. Poly is short for polyethylene aka plastic.
See the video on YouTube: https://www.youtube.com/watch?v=oDXOl4UbSqU