How to Identify an Unlabeled Yarn
Compare the yarn to others in your stash
You may have purchased the same yarn in another color. If you compare your mystery yarn to others in your stash it may jog your memory and help you identify it.
"Phone a friend"
Show your yarn to a fellow fiber lover. They may be able to identify it or know someone else who would be able to.
Take it to a yarn store
In general you should be able to tell whether the yarn is craft store quality or LYS (local yarn store) quality. Bring your mystery yarn with you on your next yarn buying excursion and see if one of the folks who work there can help you out. After all, they do stare at yarn all day!
Consult Ravelry's yarn database
Do you tend to catalog your stash on Ravelry? Maybe you've already input your mystery yarn. Perhaps you have an idea of what yarn you have and you can go to Ravelry to look at pictures and confirm your suspicions. Finally, if the ball is half-used perhaps you uploaded and labeled the project that you used it for with the name of the yarn.
Join a group of yarn sleuths
Did you know there are groups of folks who enjoy identifying unknown yarns? Post a picture of your yarn and let the power of the yarnie community work to your advantage.
Try out Yarndex
Yarndex is a large database of yarns that is useful for many things. Their advanced yarn search tool will help you to identify your yarn when you input as much info as you can. If you're not sure about the fiber content make an educated guess, and remember that you can always check the WPI yourself to help figure out the weight.
Still at a loss? No worries! Find a hook that roughly matches your yarn and get to work. Talented crocheter that you are you should be able to make something beautiful even if you're not quite sure what you're making it out of :)