We investigate dielectrophoretic deposition of single-walled carbon nanotubes using an in situ detection system. Pairs of electrodes are stimulated with a small-amplitude, low-frequency voltage superimposed on a large-amplitude, high-frequency dielectrophoretic voltage. Measuring the magnitude of the current both at dc Idc and at the low frequency Iac through a digital lock-in technique allows us to determine when a nanotube has made electrical contact and to halt the dielectrophoretic process. Because Idc is determined by nonlinearities in the device current-voltage characteristic, measurement of the Idc / Iac ratio allows the real-time determination of whether the deposited nanotube is metallic or semiconducting.