From the specifications, the LED in question is an integrated module, not a bare LED. Therefore, the Forward Voltage of the LED is not really relevant from a design perspective: Integrated current limiting circuitry, i.e. a resistor, would dissipate the difference in voltage between the actual LED junction's Vf, and the supply.
However, if this is an exercise in academia...
A diode characterizing device, or even an oscilloscope in X-Y mode plus a high-current signal generator, could be used to identify the approximate forward knee of the device's current curve. While a bare LED die would have a sharp knee, even a resistor-integrated part will show a somewhat representative curve, such as this:
Of course, if you have a few sacrificial pieces for such characterization, a more definitive range of Vf values could be approximated.
Do note that LEDs have minor tolerance variations in Vf even between parts from the same batch, and adding a current limiting resistor increases such variation.
Also, the current graph in such cases will definitely not be sharp - at best, a mild knee will be observed, broadly indicative of the underlying LED junction knee.
The specific junction chemistry determines the characteristic band-gap and light emission threshold, so if more were known of the LED in question, this would help.