The first sign of serious trouble in the heat is that those nice little beads of sweat on your forearms aren't there anymore. Dry forearms and you need to be somewhere shady where there's water inside an hour or you'll probably have a Health Event. Another thing which starts a little earlier is rising HR at the same effort. Now that I use sun sleeves all the time, I go by my HRM. I've always worn an HRM, so I'm quite familiar with what normal looks like.
I'll drink water no matter its temperature. Even quite warm water will cool one effectively as sweat. Water stays much cooler in a Camelbak, especially a Camelbak that was first filled with ice. Camelbak makes special ice trays which make long ice cylinders which melt more slowly. Even with the Camelbak, I'll have a liter bottle on the bike.
On long rides, whatever the temperature, I'll take enough Endurolytes to make me thirsty enough to drink enough so that I'll need to pee about every 3 hours. That of course has the effect of keeping my electrolytes always in balance. If that much time goes by without peeing, I'll find somewhere to sit and drink water until I pee, then go on. Even on an event ride, losing time this way is much better than a DNF.
I never drink water without thirst. IOW, I drink to thirst. That prevents hyponatremia, which is at least as great a medical risk as dehydration. Only experience can tell one how much electrolyte one needs to create thirst. One doesn't always get thirsty when dehydrated. Thirst depends more on electrolyte balance than it does of dehydration level, which can be independent of that balance. One also will have a much better electrolyte balance on long rides if one always eats a low salt diet, which sounds backwards but is nevertheless how it works.
I didn't figure this out on my own. Hammer Nutrition has a free PDF handbook which explains all of this. I read it when I was first starting to ride long distances, long ago. It works. You don't have to use Endurolytes, their brand, anything with about that composition and strength works fine. I never put electrolytes in my water, because one doesn't always need the same balance of electrolyte to water consumed, though another good way to go is to have one bottle of strong electrolyte concentration and another source of plain water, and to alternate them according to one's need to create thirst.
This was so much typing that I'm going to bookmark it for future reference.
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Results matter