The method of microwave-induced gas breakdown to diagnose the operating mode structure in a nanosecond-pulse high-power Raman free-electron laser is presented. Modes of low azimuthal and radial index, such as TE11 or TM01, are observed in the Raman free-electron laser experiments. Occasionally, some mixed-mode photographs of the output wave are also obtained, such as the sum of modes TE11 and TE01. In the region of gyroresonance and higher β1, modes with the high index of the electron cyclotron maser are obtained; combined low- and high-index modes are also observed within the same experimental parameters. The results of experimental operating mode observation are discussed and a prediction of mode analysis is made.