Here I present 3 different ways to identify any LED’s polarity. I hope this is useful for people who are starting to work with LEDs.
The longest “leg”is the positive one.
If the LED legs have been cut and are equal, then see if the LED has a flat side on the head. If so, this is the negative side of the LED.
If you couldn’t use any of the above methods, then look directly into the LED. (You must use backlighting if the LED is not High-Brightness).
You will see two plates almost touching each other. The biggest plate is USUALLY the negaive side.