Ultraviolet is the "color" that exists below the visible range of wavelengths. Because UV light is invisible, you must test the light source to verify that it is emitting ultraviolet light.
Check the UV Frequency
Check the frequency of the ultraviolet light source to determine if it should produce true ultraviolet light because some black lights produce more visible light than UV. The wavelength of visible violet light is 450 to 400 nm. A standard black light emits a wavelength of 365 nm with very little visible light. If your black light has a strong blue or violet glow, it is probably producing more visible light than UV light.
Testing True UV Light
Expose glow-in-the-dark (phosphorescent) objects to the UV light. While the UV light itself is invisible to our eyes, it reacts with phosphorescent pigments found in glow-in-the-dark objects. A properly-functioning UV light source should make glow-in-the-dark objects illuminate dramatically.
Test UV Light with Household Objects
Use household items to test your UV light if you do not have access to glow-in-the-dark objects. Exposure to UV light will cause vaseline and ripening bananas to cast a bright blue glow. Highlighter ink will illuminate, and white clothing such as socks and T-shirts will appear brilliant because of florescent dyes used to whiten the cloth.