I agree with Brian.
Another way to detect this is if you wave the light source back and fourth really fast and see multiple strings of the same light source rather than a smooth blurring of the light. Some LED flashlights exhibit this behavior as well.
Some LED lights that use variable output to give differnt levels of brightness exhibit this behavior as well. Example, if a light has three levels of brightness, the flickering may be noticable when dimmed down. Many use Pulse Width Modulation (PWM) to achieve the dimming effect unless the cirucuit is fully regulated.