Systems, methods and devices for detecting unauthorized devices at transaction devices are disclosed. A method for detecting an unauthorized device in a card slot of a transaction device may include inserting a card slot insert into a card slot for a transaction device, wherein the card slot insert has a width and thickness based on a manufactured width and thickness of the transaction device, and comprises a first indicator indicating that the card slot is secure and a second indicator indicating that the card slot is not secure. When the card slot insert is inserted to a manufactured depth of the card slot, the first indicator is visible outside the card slot, and the second indicator is not visible outside the card slot. When an object is in the card slot, the first indicator and at least a portion of the second indicator are visible outside the card slot.