You could get a necropsy on the cat?
Something like heart failure could have occured and the cat could have just died. Sometimes cats, like people, are born with heart defects, and you don't always pick it up in a young cat since they don't routinely have EKGs or Echocardiograms unless the problem is obvious upon listening to it.
Cat could have also eaten something it wasn't supposed to. Lilies, for example, are extremely toxic to cats. Animals are also attracted to rodent poison and antifreeze, both highly toxic.
I believe a necropsy is going to be the only way you find out for sure.