The Inventory Checker emits a redstone signal if the inventory behind it has more than a certain number of items in a specified slot. Both the slot and the item can be defined in the GUI, alongside an optional Item Tag, and whether to ignore or match the damage of the specified item. These can also be left blank to match any item that goes in the slot.
GUI
The GUI selects the slot, stack, optional item tag, damage mode, and threshold. Leave item or tag matching empty when any valid item in that slot should count.
Remember, slot counts start at zero!