I was fortunate enough to receive 2 bags of yarn from a friend of my mom's whose aunt passed away. The aunt was a knitter and the neice a quilter, so she shared the aunts stash with friends. Some of the yarn had labels, others no labels, therefore no fiber content. All of the yarn looks like it is circa 1970's or 1980's at the latest.
I wanted to determine which of the yarns that were not labeled were wool and which were acrylic. I posed the question to the knitlist and received numerous responses. The general concensus was that there are 3 ways to tell.
1 - knit a swatch and felt it
2 - burn test
3 - bleach test
I decided to do the burn test and confirm with the bleach test . (Okay I was a medical technologist with a degree in Chemistry and Biology in a past life.) I only needed a small jar with bleach, a lighter, and a piece of tin foil. First I took a known wool and a known acrylic and tested them. Once I knew the smell of burning hair and the look of melted acrylic, I was ready for the unknowns. I only had only two that I thought was acrylic. The rest were wool. Then I did the bleach test and the wool does indeed disintigrate in bleach. The acrylic does not.