How do I find out the cause of someone's death (if it's not in the obituary)?

Answer 1

autopsy report?

Answer 2

It's on the death certificate. But, to gain access to that document you must certify you are a relative of the deceased.
If the death was a coroner's case (death by accident, violence or poisoning) the cause might be mentioned in the newspapers as a news item.

Answer 3

The death certificate list the cause of death.

Answer 4

coroners office