Take a variable DC supply that starts from about 1.5V & give to the LED. You won’t see it lights up. Now slowly increment the voltage & see what voltage the LED starts to lights up. That’s the voltage you looking for.
LEDs operate with current, not voltage.
You must use a current-limiting resistor in series with the LED so you don't use too much current.
Then simply measure the voltage across the LED.
It will say the range of forward voltage. If it is a red LED it will say "minimum= 1.5v and maximum= 2.4V at only 20mA".
You must measure it to know what is the actual forward voltage.