halogen headlight bulbs illuminate immediately as soon as they are switched on.Xenon bulbs have a very slight delay/flicker before they reach max brightness.Also if you lift the bonnet,cars fitted with Xenons often have a yellow "high voltage" sticker attached to the front panel.If the car has headlight washers ,then this is often an indication of xenons fitted.My experience of Audis fitted with xenon headlights that fail ,it is often not the bulb at fault,but the electronic ballast unit that has gone.Not a cheap fix.hth