Testing a led
Electronic

If the cathode lead of a LED cannot be identified, place 3 cells in series with a 220R resistor and illuminate the LED. 4.5v allows all types of LEDs to be tested as white LEDs require up to 3.6v.

Do not use a multi meter as some only have one or two cells and this will not illuminate all types of LEDs.

In addition, the negative lead of a multi meter is connected to the positive of the cells (inside the meter) for resistance measurements - so you will get an incorrect determination of the cathode lead.