If an officer reports to the scene of a crash and suspects that one of the drivers is under the influence of drugs or alcohol, then they will begin an investigation by asking the driver questions. It is common for a police officer to ask a driver in a traffic accident, if they have been drinking or whether they have ingested any drugs (prescription or illegal).
During the traffic stop, law enforcement will start asking those questions to determine whether the person was possibly under the influence of drugs or alcohol. Based upon how the person is standing, speaking, what their breath smells like, what the person smells like, how they are following directions – in other words, based upon their physical appearance, and mannerisms, and their conduct, the officer may have reason to believe that the individual is under the influence. If so, then a car crash investigation will transition into a criminal and civil investigation.