A method to test a signal path with vacant bandwidth by sending a test signal twice and processing two resulting nonlinear distortion signals captured in the vacant bands to determine presence of nonlinear distortion. If the signals correlate, the energy in the vacant bands is nonlinear distortion. If the test signal is sent followed by an inverse (in time-domain) of itself, and a resulting correlation peak is negative, the nonlinear distortion is determined to have been created by odd-order nonlinear distortion.