How could unmarked diodes have its cathode leads identified using an ohmmeter?

Assuming the red lead of the ohmmeter is more positive than the black lead, then checking the impedance of the diode with the ohmmeter, in both directions, will identify the cathode as the black lead when the measured impedance is low. This, however, will only work if the voltage developed by the ohmmeter is greater than the forward bias voltage of the diode, typically 0.7 volts.