We present an indirect port-impedance characterization technique for monolithically manufactured THz antennas. Due to the exceedingly small geometrical details, standard contact-probe measurements cannot be carried out at the desired port locations. Alternatively, several contact-probe measurements are carried out at remote locations while the input port is terminated with standard loads. As such, the THz antenna is treated as a two-port network and the port impedance seen by a sensing diode can be analytically determined using standard S-parameter measurements. In particular, the diode port is replaced with three standard terminations (short, open, and a resistive load) and the measured S11 data is used to indirectly compute port impedance seen by the sensor. The initial results show that this approach provides accurate estimates of the antenna impedance, with possible exceptions when the electrical length between the two ports is close to an integer multiple of half wavelength.