Horses have complex nasal passages and sinuses that can only be fully evaluated by a vet with the use of an endoscope and other imaging. That said, you can provide valuable information to your vet regarding the state of your horse’s nasal passages and nostrils by conducting a simple assessment yourself.
Halter the horse and stand in front of them, holding the halter without exerting downward pressure. Look at the two sides of the face. Is one side of the muzzle drooping or do the sides seem symmetrical? Is there nasal discharge? If so, what does the discharge look like? Which side is the discharge coming from or is it coming from both? Keep in mind that a clear nasal discharge is normal.
Standing to the side of the horse, if you lift the inside edge of the nostril with your thumb and look (with a light source or in bright light) into the opening and onto the floor of the nostril, you will see the small nasolacrimal opening. This is what your vet flushes when they flush the duct. You should see moistness around the duct. While you are there, see the very pink mucous membrane inside the nostril. This is another "window" to your horse's health; another great place to visualize the horse's mucous membrane color.
Now place your hand in front of first one nostril, then the other. Feel the airflow as your horse breathes. You can actually get the respiratory rate here. You should also note whether there is a difference between air flow at the right and left nostril. Horses that have congestion or obstruction of the nasal passage on one side or the other will have reduced airflow on that side. Smell the air coming from each nostril. It should smell fresh and clean as should the mouth.
Tips for safety & Success
Be ready to move out of the way quickly whenever you stand directly in front of your horse.