I am not a medical person but I have been on the intimate side with some intentional overdoses, one with a friend and another with a family member. One of the characteristics of suicidal overdoses is that the suicide usually leaves a note, makes a goodbye gesture, and also ingests a huge amount of medicine, often an entire bottle, to make sure they die and can't be resuscitated. An accidental death would be more likely to be a medication reaction from more than one drug or a "just enough" situation. Heroin is often a cause of accidental overdose because its potency can be quite variable, so that the user would take what they think is enough to get high, and if they get a much purer product than they are used to, it kills them.