There are three ways to tell which side it positive or negative on an led in this fashion:

  1. Look at the long leg, that is positive
  2. There is usually a side or part of the led that is flat, that side is negative
  3. If you look inside the led you can see the larger side, which is also negative

[caption id="attachment_240" align="aligncenter" width="393" caption="3mm, 5mm, 10mm LED Polarity Image"]3mm, 5mm, 10mm LED Polarity Image[/caption]