If you had to figure out someone’s cause of death by the scene only, what would you look for?

If the coroner couldn’t determine a cause of death, so the only thing you had to investigate yourself, was the deceased mobile phone and the house where their body was found.
Apparently needles were found on scene, but no other answers as to whether or not there were needle marks, due to decomposition.

What would you look for?

Would you count the needles to see how long they were potentially injecting?
How would you determine if it was murder or self inflicted?