If it wont light up, reverse it and if it lights up then its Cathode. Touch the anode to the resistor and one lead to ground. NOTE: I took this picture myself from my camera, Please Do not mind for image clarity ,so i have put some labellings also. Connect a 330 or 470 Ohm resistor to +5v. The larger end inside the led is -ve and the shorter one is +ve, that is how we find out the polarity of a LED As they have to be crystall clear to glow so they don’t have any labeling on it, so the only way to know is either to test it with a multimeter or by carefully observing inside the led. In other words, pin 2 is the common cathode, or where the three LED cathodes join. As with the common anode above, this symbol has pin 2 as the common pin. That is, it is the schematic symbol or circuit diagram symbol of the common cathode RGB LED. Notice the picture carefully and compare it with the LED diode symbol, that should tell which one is –ve and +ve terminal. The following image shows the internal wiring of a common cathode RGB LED. I hope the figure below can solve this problem of identification. Luminous Intensity: RED color 800mcd, GREEN. Reverse current (at 5V): RED color 10uA, GREEN color 10uA, BLUE color 10uA. Forward current: RED color 20mA, GREEN color 20mA, BLUE color 20mA.

LED's or Light Emitting Diode's don’t come with any labeling on it to identify Cathode(-ve,GND) or Anode(+ve). Forward voltage: RED color 1.8 to 2.2 V, GREEN color 3.0 to 3.4 V, BLUE color 3.0 to 3.4 V. Sometime the diode symbol creates confusion too.Identifing a LED's the cathode and anode of a led is very easy by looking inside. So for identification, led’s comes with a unique way to identify its terminals as Anode or Cathode. While referring to any schematic involving led's we sometimes get confused with the identification of the terminals.

Cathode led wiring