I found this tip on Facebook and thought it was worth writing it down:
So how can you check if your horse may have sand in its stomach? The easiest way we discovered is to get a long exam glove (the ones used for rectal examinations in horses - we were able to buy ours from a veterinary clinic).
Put a few fecal balls in the glove, add water. We hang up the glove and let it settle for a couple of hours. If there is any sand, it will show up in the fingers of the glove where it is easy to see and feel.
If your horse is on sandy soil, you can do this test a couple of times a year and seek veterinary advice in case you do find sand in the glove.