Instructions
Check the legs of the LED. If the diode is new and the legs have not been cut to size, one leg will be longer than the other. The longer leg is the positive anode and the shorter one is the negative cathode.
Examine the body of a standard cylindrical LED for a flattened side. This flattened side is located at the base of the colored top, just above the point where the legs leave the body. The flattened area, called the "cathode mark," is directly above the cathode leg.
Read the codes printed around the edges of larger "block" LEDs. These LEDs, used for displaying alphanumeric characters, have a common anode or cathode. The appropriate probe is identified by the "A/C." mark. Whether this is the anode or cathode depends on whether the LED is a common anode (SA) or common cathode (SC) device.