You will notice on the spec sheet, under the heading "Feature", it says "Avalanche rated". Then a little further down is a single pulse avalanche energy rating of 8.1 millijoules.
What that means is that it's ok to use the body diode like a zener, and the device won't be harmed.
If you were to put a .1 henry inductor in series with the drain and a power supply of, say, 12 volts, and apply a pulse to the gate to turn on the device for about 33 milliseconds, the current in the inductor would ramp up to .4 amps.
Then, let the gate drive go to zero. The device will turn off, and the voltage at the drain will rise until the body diode breaks down in zener mode. The energy stored in the inductor when the device is turned off is 8 millijoules, and that energy will be dissipated as heat in the device.
What the spec sheet is saying is that you can do that without hurting the device, if the starting temperature is 25 degrees, and if you let it cool down to 25 degrees before you do it again.
For use where the body diode is conducting current continuously in zener mode, you must see to it that the dissipation remains within the Ptot rating of the device. That is, the product of the zener current and the zener breakdown voltage must remain less that .36 watts (at 25 degrees; an infinite heat sink would have to be provided to keep the temperature at 25 degrees if the device were dissipating .36 watts). If the device temperature can rise when it's dissipating power (as is the usual case), then you must keep the dissipation below the value shown in the figure 1 graph for a particular device temperature. The thermal resistance parameter, Rthjs, can be used to help determine the allowable zener current at various temperatures.
Now, as to your question about just what the zener voltage is. Typically, for MOSFETs whose body diode is specified for use as a zener, the breakdown is just a little higher than the Vds rating. I would expect it to be in the range of 65 to 80 volts. You could easily measure it. Short the gate to source, and put a 10k ohm resistor and a variable power supply (capable of 100 volts output) in series with the device's drain to source terminals with the proper polarity--positive on the drain. Slowly turn up the supply while monitoring the voltage across the device; the voltage will stop rising when you reach the zener breakdown voltage. Don't keep increasing the voltage much past the point where you can tell that you've reached the zener breakdown voltage, or you may overheat the device.
An interesting aspect of the ability of a MOSFET's body diode to act as a zener diode is that if you need a really high power zener, you can buy a high power MOSFET (some can dissipate 250 watts or more in a TO220 case), short the gate to source and use it as a 250 watt zener. You can't buy a regular 250 watt zener as cheaply as you can buy a high power MOSFET.
Also, as long as you stay below the zener breakdown voltage of the MOSFET's body diode, you can short the gate to source and use the diode as an ordinary rectifier diode. Again, you can buy a 50 volt power MOSFET with a current rating greater than 100 amps for a few dollars. You could use 4 of them to make a 100 amp bridge rectifier for a high current power supply.