In PIH an impending sign of eclampsia is:
Visual symptoms
Weight gain of 2 lb per week
Severe proteinuria of 10 g
Pedal edema
As you know, severe preeclampsia progresses to eclampsia if not treated promptly,hence symptoms of severe preeclampsia can also be called as indicators of Impending eclampsia.
Signs of Impending or Imminent eclampsia/:

Headache (severe)

Symptoms of severe preeclampsia/

Blurring of vision/flashing lights

Symptoms associated with poor prognosis

Epigastric pain
Brisk deep tendon reflexes and ankle clonus
Diminished urinary output (<400 ml/24 hours).