The justice’s personal doctor called her at 8 p.m. Saturday night. http://www.11alive.com/story/news/nation-now/2016/02/14/official-scalia-died-heart-attack/80376442/
Guevara officially pronounced him dead at 1:52 p.m. on Saturday. [] Hours earlier, the county judge told WFAA that myocardial infarction - or a heart attack - would likely be the cause of death listed. Guevara later said she would confer with the Justice’s personal physician on what specifically to show as the cause of death. http://www.wfaa.com/news/local/texas-news/scalia-to-have-autopsy-in-texas-according-to-state-law/42898331
How could the Judge without seeing the body, without an autopsy to determine the cause of the unobserved death, before speaking with the decedent’s personal physician, declare the cause of death “natural” or “myocardial infarction”?
How did she determined or confirmed the nature and cause of death?
Declaring the cause of death technically means what is put on the death certificate. According to your link she was not going to fill out the death certificate until after talking to the physician.
She is guilty of having loose lips/a big mouth in talking to the press before this. She was probably speculating on the likely cause of death based on the descriptions of the scene and events she had been told about by authorities who were there with the body. Since she is not a physician, it is not clear how it would have helped for her to view the body in person.
You can look a Texas law regarding inquests and autopsies here.
It sounds like if a physician who has recently been caring for a decedent says no autopsy is necessary that is normally what the Justice of the Peace will go with.