In our daily life, some sensitive cargos (e.g., refrigerators) are required to keep one right side up. If these items are turned upside down due to incorrect transportation or wrong storage, some unpredictable exception will happen, leading to potential economic loss. In this paper, we propose a lightweight system TagUP that uses RFID to detect the upside-down exception. By dynamically changing the frequency of the electromagnetic wave and comparing the phase difference between two measurements, TagUP is able to achieve the detection accuracy of 98.0% when two tags are placed 60cm apart.