How can you tell when a dog is over doing it at the park

I don't know I think panting often depends on the dog.  Kenya very rarely pants for more than a few minutes after exercise, and she is quicker, more agile, and needs more exercise than Coke.  But Coke will often pant for a long time.  On hot days, he really never stops panting unless he's asleep (he's panting right now, it's only 50 degrees out and our walk/park play was three hours ago).  30 minutes of fetch for a healthy adult dog does not sound like too much to me unless you're doing this at noon in a hot climate.

You have a small dog and 30 minutes is probably the right amount of exertion. There's nothing wrong with dogs working or playing until they are tired. But most any dog pants to cool off. It doesn't mean they are out of breath, they are just cooling off. Dogs can't sweat. They can only release heat through panting and through the pads of their paws. And different dogs will handle different temps depending on their needs. Shadow will lay in the snow to play with a toy or stand in the middle of a snow blizzard to eat his meal.

 My dogs won't overdo anything off leash.  If they are in harness they, well, Crusher might very well work himself to death, but I'm not sure, they do slow down.  Off leash though, both of my dogs will tell me when its too hot or they've done too much.  They just plop down every chance they get and lay there watching me. 

I'd say if it takes your dog that long to recouperate when you get home, and say she's laying there looking half dead, maybe take it easier on her.  Build up her stamina more slowly. 

If the dog seems stiff/sore several hours later you over-did it. I'm more concerned about killing a dog with heat stroke- If the dog is seriously panting with tongue really out I force the dog to stop and get in the water or shade. If it's over 50 degrees outside your dog can overheat easily.

 We get worried about heat stroke too. It is so hot down here.

 I do believe are Rosey would chase the ball until she collapses if we let her. We try to do the really active stuff in the evenings when it is cool.