Also, react as though you're angry at your accuser. Imagine you're pulled over by a cop and he asks if you have drugs in the car. If you didn't it would be natural to get a little angry at the supposition that you're somebody who uses drugs. Also, since you're innocent and have the truth on your side, you wouldn't necessarily be worried about the cop pulling you out and searching your belongings on the side of the road. Somebody with drugs in the car would be worried about this and would react pretty differently to the question in general.

How a person reacts when being accused of something can really tell you a lot. They may lie outright, obfuscate the question itself, turn the question around on the accuser (as OP's example illustrates), etc. 9 times out of 10 though it's pretty easy to see when a person is trying to concoct a story. In high pressure situations people are rarely as good as they think they are at thinking on their feet and in my experience, they're almost never as good at lying as they believe themselves to be.