A method of testing distributed amplifiers is presented; multipath interference (MPI) is detected as a beat spectrum between a multipath signal and a direct signal using a frequency-modulated test signal. A test signal with an approximately 450 MHz frequency deviation at an 80 kHz modulation frequency is emitted from a directly modulated DFB-LD by a pulse stream passing through an equalizer. A receiver consisting of a photodiode and an electrical spectrum analyzer (ESA) detects a baseband power spectrum peak appearing at the frequency of the test signal frequency deviation. MPI is converted from the spectrum peak power using a calibration chart. The test method has decreased the minimum detectable MPI as low as -70 dB, compared with that of -50 dB of conventional test methods. The detailed design and performance of the proposed method are discussed, including the calibration procedure, computer simulations for evaluating systematic errors caused by the repetition rate of the frequency modulated test signal and the fiber length under test, and experiments on single-mode fibers and distributed Raman amplifiers.