How will I know if my horse is getting overheated? Down where I live the summer's are around 90 degrees with extremely high humidity. It makes me nervous to take a horse out riding in such heat.

A quick search tells me that some common signs of overheating for horses are muscle tremors, heavy sweating, and even a darker-than-usual color of urine. I would think that you would also be able to tell in the way your horse is behaving. I'm sure that all horses are different though, and if you know your horse well enough you can probably tell when something is amiss.

I would think too that simple measures such as making sure that there is access to shade, and to ample amounts of fresh water would help to offset the risks of overheating.

I think that FluffNStuff is right. If you take your horse out, you should be sure to map out a path that has water or bring some of your own so you don't risk this. Give plenty of time for breaks in the shade.

Yes, Sheraton and Fluff both have it right.

Make sure to not forget about yourself either. My advice to you is to take along a canteen for yourself and another for your horse. Of course you will need something to put their water in ;)

The things you are most likely to notice is the excessive sweating and muscle tremors and a general weariness. Sweating starts around tack (bridle, saddle pad, etc) but can often start moving onto the neck, haunches, and chest with heavy work. Once it starts covering the entire horse you are getting toward sweating more than you want to without adequate rest and water after. Which means for trail riding its not good.