Police can't simply pull any driver over and carry out sobriety tests. They need to have probable cause to suspect the driver of breaking the law before they pull him or her over. This probable cause doesn't have to initially relate to drunk driving. It might simply be that the driver was speeding or drove through a red light. However, such an innocuous stop could later -- after police examine the driver -- lead to a DUI investigation and arrest.
Here are three types of key evidence police might observe in order to suspect a driver of intoxicated driving: