I bought a second hand twin plate oz giken clutch and the mechanic i went to just compared it to another one he had there with half decent plates on it. If there is another decent plate you can compare it to with some vernier calipers go for it, otherwise just make sure it's not paper thin.
If i remember correctly, put the thiner plates furthest away from the flywheel to make it last as long as possible.
edit: beaten dam it!!