Krupski:
danb35:
Measure it with the diode setting on your DMM. If you don't have a DMM with a diode setting, you really should. Or, do as otherwise suggested here--pick a resistor value, wire it up, and then measure it.I made myself an LED tester. I use it all the time. It's almost indispensable. It's just a current regulator set to 20 milliamps and a 9 volt battery - no power switch... and I can measure the forward voltage of any LED just by sticking my voltmeter across a lit LED.
How did you deal with possible reverse polarity connection to the led? (retorical question, you didn't deal with it per your drawing) 
Most standard leds have a maximum reverse voltage rating of only 5vdc. An internal series diode in series with the test leads output would probably be enough.
Lefty