In an attempt to figure out if some sweaters are really cashmere when there is no tag, I have been working for the past year on educating myself to identify fibers using a microscope. There are other tests that I can use too . For example, wool completely disintegrates in bleach - and burns completely to a fine powder. Acrylic, on the other hand, does not completey dissolve in bleach - and leaves a hard black bead when burnt.
The problem is cashmere is an animal fiber like wool, and therefore reacts to many of those tests the same as wool, and close enough that it is not significantly different. Thus, the microscope.
Wool has many "scales" - under the microscope these look like diagonal lines that intersect and they are pretty close together. When put in hot water with a little soap and agitation, those scales open up (the little points flair out at the edges) and they can get caught up with the other open scales of adjacent wool fibers and attach. When the fibers cool down, the scales tighten up and voila! you have felt.
Cashmere doesn't have the intersecting lines like wool, thus is won't felt - at least easily - or at all. The two fibers (wool and cashmere) do look different under the microscope - and can be pretty easily identified. At least I haven't been stumped yet.
An even more obvious difference is between animal fibers and synthetic fibers (acrylic). Since these are essentially extruded when they are produced, they have vertical lines instead of horizontal. It's really hard to confuse synthetic and animal fibers under a microscope.
So, if you still don't believe me - or if you want to try felting wombat or horse hair - check here, the Australian Commonwealth Scientific and Research Organization (CSIRO) has put out a publication on fibre analysis - and page 5 has a chart with photos of all different fibers :)