What are first signs of a sunstroke?

Heat stroke: Heat stroke can present with high temperature (104f), inability to sweat, flushing, strange behavior, high heart rate, abnormal breathing, seizures, and even coma or death. It may or may not follow heat exhaustion characterized by nausea, vomiting, malaise, dizziness, cramping. Heat stroke is a medical emergency. Hydration and avoiding exertion in hot temperatures may prevent it.

Stopped sweating: Sunstroke occurs when you get dehydrated and run out of sweat, so that your body's thermostat can no longer keep your body cool. Your body relies on water evaporation to stay cool. The first sign is sudden cessation of sweating; that means it's time to get in the shade immediately and drink lots of fluids!