Monitoring and assessing instrument performance and response are crucial to various seismological analyses that utilize the seismic signal recorded by the instrument. One of the important components of the instrument response is the gain or the amplification factor that determines the amplitude of the recorded wave arrival. We introduce a new method to detect problems in the gain of three‐component seismographs by examining the body‐wave polarization. Anomalous gain of a certain component causes P‐ and S‐wave polarization to be distinct from expected values, allowing one to identify the issue in the instrument. The method is applied to the High‐Sensitivity Seismograph Network (Hi‐net) stations between 2004 and 2016, and 305 out of 790 stations are identified to have issues at various time periods. The detections are confirmed by comparison with the Hi‐net daily calibration pulses. Utilization of teleseismic body‐wave polarization information is an effective way to detect instrument gain problems without physically examining the instrument, which is particularly advantageous for instruments such as borehole or ocean‐bottom sensors that cannot be accessed easily.