Halter the horse. After putting on latex gloves, use one hand to lift the upper lip, and look at the shape of the incisor teeth. Are they longer than they are wide? What angle do they form with the lower teeth? Is it an acute (sharp) angle or do they meet near right angles? Does the horse have an overbite or do the teeth meet evenly?

Look in any available horse's mouths for which you know the age. Think about these two characteristics: shape of the tooth, and the meeting angle. Learn to tell baby teeth from permanents. Young horses have fairly predictable eruption times for the permanent teeth. These can be used to age with some accuracy in the younger horse.

When you really want to age a horse, take a photo of the horse's incisor teeth from the side, and the front, and try to photograph the grinding surface of the lower incisor teeth.

Send these photos to your vet, or better yet, have them examine the horse. They can help confirm (or refute) your hunches about the age of the horse.

Be careful not to be bitten when you are looking in the mouth. Never try to open a horse's mouth when they are wearing a tight halter. They will feel pressure on the nose band and will resist.

If you use a head light, turn it on pointed away from the horse, then slowly swing it over to the mouth.

Looking in a horse's mouth should not be a struggle. If it is a struggle for you, either you are not doing it correctly, or the horse has not learned the cues that you are giving. Either way, it is best to stop and get help.

How the teeth "age" depends somewhat on the specific horse, and how their teeth meet and wear. These days, with motorized equine dentistry being as common as it is, it is not unusual to see horses with incisors that have been ground down, completely changing the tooth appearance, thus their presumed age. Some horses age differently on the left and right sides of their mouths.

Keep in mind that recent studies have shown that the "science" of horse aging can be very unreliable. Likewise, memorization of aging diagrams is of little value. There is no substitute for first hand experience. Practice aging your own horses and those of your friends.

There is a rare but potentially fatal possibility for you to contract rabies from a horse infected with this disease if their saliva comes into contact with a wound on your skin. Because of this, it is recommended that you ALWAYS wear latex gloves when examining your horse's mouth.