Hawaii x Reference Guide (HxRG) detectors have crosstalk between amplifier channels at a scientifically relevant level. In principle, crosstalk signals can be fully calibrated and removed from data but only if a full crosstalk matrix is measured for the detector. We present a fast method of crosstalk characterization that can be performed with most instrument calibration units. It requires only a flat-field illumination and window programming in the HxRG detectors. We show the crosstalk matrices obtained with this method for both fast and slow modes in an H2RG detector and provide examples of how this data can be used to tune detector operation parameters, for feedback into the electronics design for the cryogenic preamplifiers, and in the data pipeline to remove crosstalk signals from scientific data.