First of all, you need to spend a lot of time with your horse which will help you to know its mood and physical condition. In this way, you will immediately know if anything unusual happens to your horse.
Make sure you spend a few minutes in examining the physical condition of your horse every day before taking a ride. It will help you to know if your horse is physically good or not.
You also need to learn how to check the heart rate, temperature and respiratory rate of your horse which will help you to examine the physical condition of your horse without taking help from a vet.
If your horse is acting slow or dull which is unusual to you then you should understand that there is something bad that is affecting to the health of your horse.
If you notice that the appetite of your horse is decreasing then it is an indication that something is wrong with your horse. Keep observing if it is eating or drinking properly or not.
You should notice if your horse is facing any discomfort like biting or it avoids placing its foot on the floor. It indicates that some sort of pain is causing discomfort to your horse.
Make sure you keep noticing the routine habits of your horse which will help you to determine if anything unusual happens to your horse or it starts behaving in a different or strange manner.
You should use rectal thermometer for checking the temperature of your horse because it will become very difficult to judge the temperature from mere observation.
If you do not understand the problem with your horse, then you should immediately call for a vet who will help you to find out the reason that is causing uneasiness to your horse.