1, Looking down from the top of the LED there is usually a flattened edge, this identifies the negative pin of the LED.
2, If the LED is new, there will be one lead longer than the other. The long lead is the positive pin and the short lead will be the negative pin.
3, If you hold the LED sideways on and view through the plastic, there will be two parts, a small straight pin and a fatter 'L' shaped pin. The small pin is the positive side and the fatter 'L' shaped pin is the negative side.
My picture illustrates the three possible ways to identify an LED and also shows the polarity as would be drawn in a circuit diagram.