Various methods and devices transfer test patches of marking material from a marking device of a printing apparatus to a transfer surface of the printing apparatus, optically measure the density of the test patches on the transfer surface using an optical sensor of the printing apparatus, and measure the electrostatic differences in charge of the transfer surface as the test patches on the transfer surface move by an electrostatic sensor of the printing apparatus. Such methods and devices adjust settings of the marking device based on output from the electrostatic sensor alone, or based on a combination of the output from the optical sensor and converted output from the electrostatic sensor.