[evil6remlin]

if you look at his engine look for a hose of some sort (most likely steel braided)hooked up to his intake somewhere where the nos would be sprayed if he has a dry kit. And I think wet kits have special injectors that spray fuel and nos at the same time (I'm not sure about this) so you could try to look around his fuel rail and injectors for anything out of the ordinary. Other than that you could play it smooth and seem interested about the car and ask to look around the inside and then look for a bottle in the trunk. Either that or just like Drifty said just set up the rules of the race and say "no nitrous".