The invention provides methods and devices for estimating power amplifier nonlinearity using simple correlation techniques. Methods and devices of the invention can monitor a power amplifier that has digitally modulated inputs and an output containing more than one signal stream. A preferred method of the invention creates a test signal by forming the products of several pseudorandom noise sequences from the digitally modulated inputs to the power amplifier. Nonlinear contributions of the power amplifier output are determined by cross-correlating the test signal and the total output signal of the power amplifier. In preferred embodiments, the determined nonlinear contributions of the power amplifier are used to introduce corrective predistortion in the power amplifier.