If you have scavenged the LED (like I do most of the time) the longer (positive) side has already been cut. You can tell (99.9% of the time) which side is positive and negative just by looking at the contacts inside of the LED.
Usually there are two pieces of metal inside an LED, the bigger one (usually with a 'bowl' shape at the top) is the negative while the smaller, longer one (usually with a VERY small wire going into the 'bowl') is positive
Doesn't work ALL of the time, but I'm sure you won't encounter any problems...
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in 5 mm leds there is cut side on the base of the plastic. this side is -
in new leds the longer wire is +
try with battery and resistor and see. wet finger is subsitute for the resistor (and good one for blue or white leds)
in most leds the cup is - snd the stick is + (not s rule though - there are lots of leds where its the other way)
thank you it works yea