how to tell how much voltage to give an LED

i have a bag of leds i bought for somthing a long time ago, but i totally forget what size resistor to run with them. is there a way i can check this with a multimeter without just blowing out a bunch of them till i get it right?

[url]http://www.luxeonstar.com/resistor-calculator.php[/url]

hmm ok, i want to run them on 12 volts, and im guesing 3.5 volts for a 3mm white led? does that sound accurate? and i think most leds take about 20 milliamps?

They should have come with a data sheet giving this info. Some LEDs take alot more current than that, and between 2.5-3.5v