Short-pulse lasers are used to characterize the nonlinear response of amplified photodetectors. Two widely used balanced detectors are characterized in terms of amplitude, area, broadening, and balancing the mismatch of their impulse response. The dynamic impact of pulses on the detector is also discussed. It is demonstrated that using photodetectors with short pulses triggers nonlinearities even when the source average power is well below the detector's continuous power saturation threshold.