For the most part, natural human hair is a better choice for wigs and hairpieces, though it tends to be a bit harder to come by. If you're in doubt about the material that was used to create a hairpiece you own or might be considering for purchase, there are several ways to tell what kind of hair went into the piece's construction.
View the hair under a microscope. If the hair is synthetic, it is made from a form of plastic that will appear clear when lit and magnified under a microscope. Natural hair that has never been dyed will have a solid core and clear outer layer, while dyed natural hair will have solid color throughout.
Expose a strand of hair to flame. If the hair is natural, it will immediately catch fire and burn nearly instantly, curling into a ball. It will also have a strong, somewhat unpleasant smell. Synthetic hair, on the other hand, will slowly melt rather than burn, and will have almost no smell.
Look at the color. If the hair has subtle changes of color and highlights, chances are good that it's natural and has acquired its color through exposure to natural light. This look is difficult to achieve through artificial means, so its presence may indicate natural hair.
Feel the texture. Each strand of natural hair is coated with scales. This means that the hair is smooth if you pinch and rub it moving downward, but resists and feels different if you rub it the other way. Synthetic hair is smooth whichever way you rub it.