Captured: 2018-05-12 at 09:24 from forum.digikey.com
When looking at a through hole LED this often built into the physical characteristics of the LED. Here we have a diagram from Broadcom that shows the Cathode on their part . This is shown in two ways on this part. First when looking at the base of the lens. The flattened side is the cathode side.
On that same part you can also see that the leg on the Cathode side is slightly shorter. While this is not a standard these are both common practice. You should be able to verify this in the data sheet.
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Here is a picture of an LED that I have labeled the same.
