There are many problems doing tests on witches. They often won't sit still frequently turn into cats or other animals, equally turn those conducting the tests into newts, roaches or stones. And if they can get their hands on a broom they tend to pop off to the tropics for a pinna colada (or s that werewolves?).
I've never heard f a standard for witches but I suppose they would involve their abilities to cast spells, ride brooms and turn people into newts.
Tensile, elongation, area reduction?
What you need to find out is whether they float and are made out of wood.
The best test is to compare their weight to a duck. Though I suppose any waterfowl might do.