I are a vegetable.
I recently found several unidentified balls of yarn lurking in my stash. How can I figure out what they're made of? I've heard you can light yarn on fire to determine if it's wool, but I'd rather not risk a raging inferno.
Flammable in Fairfield
It's true that setting fire to yarn can help identify its content--wool is self-extinguishing and will turn to ash, while synthetics will melt, rather than burn. But as you point out, you run the risk of lighting your home on fire--and kittehs do not like fire. Still, if you take the right precautions, a burn test can be useful. Cut off a strand of yarn, place it on a non-flammable surface (ex: brick), light the end and see what happens! Burning wool smells like burning hair, cotton or a plant fiber will smell similar to paper, and synthetics will of course smell rather chemical.
Another less risky idea is to knit up a small swatch and run it through a few tests. Does it felt in the washing machine? Then it's probably mostly wool (and not superwash). Does it refuse to block flat? That's probably a synthetic. Even if you're not entirely certain, now you have enough information to care for any garments made with the yarn in question, because you know how to wash and block it.
If you don't want to go to the trouble of knitting a swatch, you can also try spit-splicing the yarn--cut a strand, pull the plies loose and wet them, then try to felt them back together by overlapping the ends and rubbing them together between your hands. If they hold, you have wool; if they don't, acrylic.
Of course, superwash wool won't (or shouldn't) felt, so you may still be fooled.
You can also try the bleach test--bleach will dissolve wool, but not acrylic. Leave a strand of your yarn soaking in bleach overnight, and see what you find in the morning!
The good news is, even if you never figure out what your yarn is made of, your kittehs will still love playing with it and/or chewing on it.
Have a purrrrrrrrfect day!