just use a dmm , one 3.3K resistor and a 12V supply to check out. Connect the 12V + supply to resistor. other side of resistor can be connected to the cathode of diode (marked side) the second side of diode can return to -ve of supply.
use DMM as DC voltmeter and measure across the diode
if it is zenor of less than 12V type, the value would be read
if it is ordinary diode, it would not conduct at 12V, and the meter reads full supply voltage.
this solves you problem rather than imagination.