I'm no expert, but going by previous cases there are a few things off the top of my head that the post mortem might have shown which could indicate a non-natural death:
- She didn't drown (ie was already deceased when entered water)
- She had blunt force trauma injuries caused in life
- She had stab / gunshot / other obvious "murder" wounds
- She had a fractured hyoid bone (neck) which is a strong indicator of strangulation and not commonly done accidentally
- She had ligature marks on her neck
- She had other flesh or bone wounds
- She had traces of poisoning (though tox reports would not be back yet)
I'm sure others will have alternative ideas to add to this macabre list! I'm assuming some of those will be harder to ascertain given the length of time Libby was in water, but it's amazing how they can determine whether injuries were caused during life or after death.
I am hopeful that forensic ecology can determine which bits of water Libby has been in. For example there could be traces of mud/plants in her lungs or stomach which are only found near the park part of the river Hull.