But I wonder: Can someone be convicted of DUI and manslaugher ex post facto when there was no BAC test at the scene of the crime, and the police are going only on hearsay about his "drinking the same amount" as his passenger?
Yes. A BAC is not required to charge someone with DUI. The police will have to rely on witness testimony of erratic driving as well as the defendant's appearance, including smell of alcohol and slurred speech, and actions following the accident. Evidence of flight after the accident and warning bystanders not to call police, especially when there is a fatality of a child, gan support a finding of consciousness of guilt. If the circumstantial evidence is viewed strongly enough, a jury can return a guilty verdict on DUI. These DUI cases are not easy for the prosecution to win.
The State may have a better chance winning on manslaughter if the reckless driving can be objectively established by witnesses.
