Ultraviolet LEDs produce ultraviolet light which your eye can't see. This is how I tested to make sure that the UV LEDs I bought were working correctly.
Here's what you need:
Transitions lenses use a special photochromatic coating that responds to Ultraviolet radiation. The stronger the UV radiation the darker the lenses become. I have a pair of photochromatic glasses so I used them to test out my LEDs.