when you see them normally having ”white sweat” and not just when they are in high work
I live in Florida and just try to avoid riding from 11 and till 3 pm. If you are worried about it just do a short (30 min ride). It sounds kanda crazy but if you get the Gatoraid powder sold at the store and buy 2 scoops in and fill with water. The horses really enjoy it and it seems to cool them down pretty quick
First of all you need to know that every horse is different and has a different way of adaptation. It also depends of the horse's breed (a thoroughbred is more used to heat than an icelandic horse for example).
In general, horses tolerate more the cold than the heat, the best temperature for a horse is between 5 and 15°C, they become to suffer a little above 20°C and start acting differently than usual above 30°C. But naturally they live outside so they are used to it and know how to avoid the heat.
What you can do is riding at fresh hours such as early in the morning or late at night (avoid the hours between 11am and 5pm), also when there is wind, it will be easier for your horse to tolerate. Choose in priority walks in the forest (look for shade) or short sessions (30mn) with a lot of walk and divided work with a lot of pauses. Don't make a long warming up but favor recovery.
It depends of the horses and his physical conditions, if you have a competition to make you need to be prepared to work under heat.
Always shower him after the work. You can also refresh him during the work with wet towels or even bottles on the neck like in endurance.
Don't forget to hydrate him a lot (without enough water he can make colics, be careful that the water is not too cold because it could make thermal shock) and give him stone salt (he will loose mineral by sweating). You can also complement his ration with electrolyte after the effort.
You need to be really careful for young horses that are not used to it yet and also old ones that are more fragile.
I hope i could help you :)