To check for airflow in the horse, put a small mirror in front of his nose, and watch for the degree of fogging caused by airflow from each nostril. To check for fluid in the sinuses of a horse, open his mouth slightly and tap above the facial crest (ridge below the eye). You should hear a hollow sound like a coconut on both sides if they are normal. A dull sound may indicate fluid in the sinus.
For more information, please read the article Sinusitis.