There are several way to test for hydration. The skin tent is one, Capillary refill is another and gum color is a third.
For the capillary refill, press your thumb hard against your horses gums, After a moment, remove your thumb and count how many seconds it take for normal color to return to the tissue. Compare that time to what is normal at home when the horse has not been worked. Ie If it takes 1 second at home for the capillaries to refill but takes 3 seconds on the trail The horse is becoming dehydrated. Comparisons like this are based on you KNOWING what is normal at home.
Gum color is also an indicator. Usually gums are pink because of the blood flowing thru them, A dehydrated horse has reduced blood flow. And the gums get more of a grey color.
Also you can press your thumb against your horses juglar vein. And count how many seconds it takes to fill the vein. Again compare to what is normal when the horse is not being worked.
Gut sounds are another sign. If you listen to your horse's gut, You can almost always hear gurgling and bubbling as material flows through his gut. As a horse become dehydrated. The material in the gut slow or stops.