This is why autopsies are a thing. The LA County Coroner is one of the biggest in the nation and their forensic pathologists have written a number of textbooks on various subtopics. Generally speaking, forensic pathologists follow a standard process specifically to make sure they avoid missing things. That said, drugs on board has a high rate of being classified as an accident. Not suicide unless they left a note; not homicide unless there’s a suspect or there’s something about the situation that precludes it (e.g. opiates on board but also gunshot to the back of the head).