Pull the car up against a wall.
Switch on the lights, they will come on instantly with a slight flicker, the top cut off will have a blue tinge to it. The light will have no yellowness to it like a normal halogen.
Similarly, when you turn them off, they will go out instantly.
Compare the light to your fogs, if they are different colours, they will have a high probability of being xenon.
Neal beat me to it...lol