PROCEDURE
Take two fecal balls off the top of a pile of fresh manure (do not pick up soil with the manure) and place them in a large heavy plastic bag. Fill the bag 1/2 way with water. Break down and completely mix the manure in the water.
Hang the bag up so that the sand is allowed to settle to the bottom of the bag - tilted to one side so that the sand settles in a corner. After several minutes, visually examine and feel the bottom of the bag for sand. A few grains are not indicative of much. However, a layer of sand suggests that the horse has a fairly large sand load. Share your findings with your vet.
TIPS FOR SAFETY AND SUCCESS
If possible, it is best to use a clear exam glove for this procedure because the sand settles into the fingertips of the glove, making it easy to feel. That said, a plastic bag that is tied off and tilted (so the sand settles in one corner), works fine too.
Note: While this test may be helpful, sometimes it can be misleading because in some cases, horses with large quantities of sand in their intestine do not shed it in their manure.