This function can be used in the BP_EquipmentInventory and BP_PlayerInventory by searching for HasItemEquipped. It allows you to input an item ID (Row name of item from the MasterItemList) and the function will return if that item is equipped and the equipment slot it is equipped in.
Go to the BP_EquipmentInventory and create a new function called HasItemEquipped and add a new input, call it ItemID and set its type to Name, then add an output called ReturnValue and set it to a Boolean. Last add another output called EquipmentSlot and set its type to S_EquipmentSlot.
Add the highlighted nodes shown in the image below, location at the top of the image.