For those of us with old stashes it is a good thing to know if there is poly/cotton in the stash. Poly/cotton was really prevalent in the late 1960s through late 1970s. Personally, I can tell by ironing the piece in question. I have an highly sensitive sense of smell. If I set my iron on a cotton setting and iron a piece of fabric that is poly/cotton the smell of the fabric changes as I'm ironing. I got in the habit of testing the fabric when Auntie started emptying her stash into mine. It is also a good idea if you frequent thrift stores for your fabric.
For a visual on a burn test go to this article from Sew News.