Detail, Kangaroo Leather
Here are some simple tips for identifying the tanning method used for a sample of leather.
Wet a piece and then stamp a mark into it. If the mark has a crisp impression afterward, the leather is vegetable tanned. If the impression is just discernable, it is chrome tanned.
An easy way to test whether leather is 100% vegetable tanned is to drop a small strip of the leather into boiling water (be careful). If it is 100% vegetable tanned it will instantly shrink and curl up like a worm. If it doesn't other tanning agents have been used in tanning the leather.
If the leather is a chalky white color, it is probably alum tanned.
Chrome tanned, undyed leather is a pearl grey with a faintly bluish tint. Dyed chrome tanned leather often retains a grey blue or white center core. Beware of chrome tanned leather for knife sheaths - it can corrode the metals.
Period Leather - an article on period leather types, identifying, and tanning.
See SkinDeep - The biannual Newsletter from J. Hewit & Sons Ltd. for good articles on leather manufacture, particularly bookbinders leather.