There are several ways to assess your horse’s hydration status.
First, determine their capillary refill time (CRT). To do this, lift their upper lip, press your thumb to their gums and watch for the area where you pressed to turn white. Count how many seconds it takes the area to return to their normal pink color. If it takes longer than 2 seconds, then your horse is likely dehydrated.
Also, while you are looking at the gums, feel the membranes and check to see if they are moist or tacky. Tacky mucous membranes are usually a sign of dehydration.
Another way to test your horse’s hydration status is to pinch the skin of their neck between your thumb and fore finger and then let it go. If it takes longer than 1 second for the skin to bounce back to normal, then your horse is likely dehydrated.
The last way for you to test your horse’s hydration status is to look at their eyes. Dehydrated horses will often have eyes that are sunken or more deeply set into the skull than usual.