In a testing circuit performing a testing operation to detect an RF circuit characteristic, a first filter unit is provided, having a first external terminal electrically coupled to a testing signal and a second external terminal electrically coupled to an RF circuit of the RF device. The first filter unit is configured to allow the testing signal to enter the RF circuit while blocking an RF signal transmitted in the RF circuit from entering the testing circuit. In addition, a testing-result informing unit is provided, having an external input electrically coupled to the first external terminal, and generating an informing signal, which indicates a condition of the RF circuit according to an electric level at the external input.