The Inventory Tester must be placed on an inventory, and stores an item in itself.
It will emit a redstone signal if the inventory is capable of accepting that item.
It also checks the side of the inventory it's attached to. If placed on top of a Furnace, it will only work if the Furnace can accept the item into its top.
Because of that, you may want to also use the Inventory Rerouter.
You can invert it in its GUI, so it emits a signal when the inventory can't accept the item.
