Can they only diagnose acute fatty liver of pregnancy if you die?

No. Biopsy(a very skinny needle)into the liver can confirm this diagnosis of liver disease.
No, no, no. Although rare, it's a very high risk diagnosis for both mom & baby. Laboratory tests, exam, clinical course, & physician experience are crucial to diagnose the lethal condition, but death is not required to confirm the diagnosis. If aflp, need delivery & an ICU setting to preserve affected organs & for full recovery.