Jimmy, very sorry for your loss. I am working on my first retriever, and he is a part of the family. I don't know what I would do if something happened to him. If it is any comfort, it appears to me that your dog went out doing something he loved with someone he loved.
I am very lucky to have a training mentor who has a lot of experience in the dog games and who is basically a professional risk manager. He has drilled into my head that I have to be the brains of our outfit, because all my dog wants to do is retrieve. So, to the extent that the above is true, I hope we all learned a lesson from the gut-wrenching experience that Jimmy had the wherewithal to post for the world to see.
To the extent that we are all doing things like hunting our dogs where they may be killed by a wolf pack or taking a dog on a dove shoot when it is 102 degrees, maybe we want to think long and hard about what we say about it. Given karma's tendency to be what she is, I can only hope that she did not take notice of corny's statement that he never had a dog succumb to the heat and never will. If only because I don't want the poor dog to suffer.
That said, your dog's tongue and eyes will tell the story most of the time. When the tongue gets wide and flat, and particularly if it starts to curl up on the end, your dog is getting hot. If you notice the eyes getting red, the dog needs to be cooled off ASAP. You can do this with cool water, particularly on the underside, and the pads of the feet. I keep rubbing alcohol with me because it evaporates faster and thus cools faster. If you are doing either of these things, a vet visit is needed ASAP, because you can actually cool the dog too much too fast and really cause some problems.
Remember too that humidity has at least as much to do with this process as the heat itself, and I stay concerned about this in Alabama where I live. When the dog is panting, the breath coming out is essentially at 100% humidity. The more humid the outside air, the more difficult it is for the evaporative cooling process to work, so heat builds up more quickly.
I am fortunate to have a stainless dog box with a fan and water, so I have some things with me at all times. It is the very rare occasion when the temperature is over 60 degrees that I do not have ice with me. If I expect it to be over 80, I will likely have several frozen water bottles with me to give water from and to cool down in a hurry if it gets to that point, God forbid. I have yet to need them, and I have been laughed at several times, but then again I won't need them until I need them.
Again, Jimmy, sorry about your loss, and thank you for having the courage to post this here.