In Texas we really have to be careful because six months of the year it's deadly hot here for black and tans like my Zeus.
It's easy to tell when he's getting too hot on a walk (the sun makes his back almost too hot to touch). He becomes lethargic, seems distracted, has his head down and ears back and almost needs to be dragged along with the leash. It's this change in his attitude on the walk that keys me to give him a break.
I always carry water for him. But I'm careful to not let him drink too much during each break.
When playing fetch it's tongue and the frothing at the mount that I watch for. When he's played out his tongue will hang almost to his knees and there will be pronounced frothing at the corners of his mouth.
I'm lucky, in that during play he'll sense he's had enough and take a break in the shade to recover his breath. If he's no where near water he doesn't recover enough to play but for a few more minutes before it's clear that we need to call it a day.
But if he has access to puddles or ponds or streams he'll dive right in and wet his feet, legs and belly too. If he has access to one of these
Rubbermaid 4243 50 U.S. gallon Capacity Stock Watering Tank @ Rubbermaid For Less
He rapidly recovers and can play much longer. But I still limit his play to what's reasonable.
BTW, I bought one of the stock tanks in the link above on Sunday to replace our series of plastic pools that Zeus plays with for a week and then chews up. No way he's gonna chew that one up.
And, it's small enough and sturdy enough that I can just tip it over with two hands to drain it when it gets dirty. I used to have to bail the plastic pools out until there was just a little water in them and then tip them, or, they'd bend and split due to the pressure.
I can't recommend them enough.
BTW, I paid only $59 for mine at Tractor Supply here in Texas. I'm sure most retailers of farm and ranch equipment offer them for the less than the rubbermaid add cites above ($129).