This is a really nice project for anyone who is into recycling old electronics. You don’t exactly get a data-sheet when you tear open that old VCR and snag a bunch of LEDS.
Normally, I set my variable power supply to 5 V and attach the LED via alligator clips in series with a potentiometer (variable resistor). The potentiometer is turned until the desired brightness is achieved. Then, the circuit is powered off to measure the resistance of the potentiometer using a multimeter. If I’m concerned that the LED is being driven beyond specification, the multimeter can be hooked up to measure current with the power turned on.
Make your own LED Test Tool