I just got this off a website. Here's the link:
http://www.austindogzone.com/?p=1408
It says the first signs are
• Restlessness • Panting • Increased respiratory rate • Increased heart rate • Excess salivation • Vomiting • Diarrhea
Make sure you are keeping her well hyrdrated. Try including an electyrolyte supplement during periods of extreme workout or sweat. Check out these supplements made for sport dogs- Rehydrate Electrolyte Sports Drink for Dogs and PSCPets.com IntelliLyte. Beginning training in the warm weather may be too much for some dogs. Pay close attention to the warning signs of heat exhaustion,such as excessive panting or nausea and vomiting. An effective and healthy training schedule will be the best for your dog in the long run. Good luck!