The Item Sampler is a block that holds exactly one item, and lets you measure its maximum stack size.
Taking a redstone comparator output provides an output relative to the max stack size of the item, where a non-stackable item outputs a signal strength of 15, a 64-stackable item outputs a signal strength of 1, and all others lie somewhere in between.
The exact equation for signal strength is round(17 - 2 * sqrt(MAX_STACK_SIZE))
