I have trialed in the heat. Usually I just go for it and see how Koji is holding up. We've trialed in dry heat that's over 105. BUT, we usually don't go anywhere unless I'm under shade. So, if I'm waiting to run, I'm under shade or I don't leave my tent. Once, Koji finished his run, his tongue was so fat and long it was hanging out the side. I was very worried.
But, I put him in the kiddie pool, wet his paws and tummy (as far as I know, you shouldn't wet your entire dog, because many times, our dogs retain the heat as the outer coat dries, but someone can correct me if I'm way off base), gave him some treats (as he hates the pool), and carried him back to the tent so they'd stay wet.
I have also bought those cooling mats. I will resoak it in the water from the ice chest, so it's VERY cold.

I also have the Ryobi fans that I don't think they make anymore. They're VERY expensive (well, the fan is like $40, but the battery can be $100), but they work well.
If Koji is acting off or very lethargic, I won't run. I try to really keep an eye on him and gauge if the heat is making him tired, or if he's just not doing well and is getting loopy (bad sign).
Lastly, if you do trial during summer, don't forget to take care of yourself too. This woman's first experience had her throwing up (heat stroke) after her runs. Can't believe she ran either, but she went for it. Tons and tons of water. Keep the dogs shaded as much as possible. Buy a cooling mat, or even those reflective coats (I got one at a raffle, Koji looks like a space cadet), can't say if it works as I haven't really had to use it. Fans are great. Keep their paws cool and tummies.