A driver was going into a bar with a drunk friend, and they were laughing and probably stumbling a bit due to the driver's helping the drunk friend into the bar. Somebody saw the car before they stopped at the bar, and called the police to report the car as being operated by a drunk person. The police came to the bar and questioned the driver, then they gave him a breath test which was above the legal limit. He is now charged with DUI. Since the man was drinking at the bar before the police arrived, how can they prove that he was drunk when he was driving the car?