The Symptoms we had:
Colic
Visible Bloating
Stiffness or lack of control of hind end
Lethargic
Anorexia
Death
Significant loss or gain of weight
Change in behavior
Dragging feet, to the point of squaring of toe
Homotobins, with no sign of injury
When blood was drawn by vets, our horses results came back abnormal.
The horses that died immediately after comsuming Monensin usually had not show signs of exposure, however horses that die in weeks or months later do show lesions on their heart. Infortunately there are no test that can be done to determine how much exposure a live horse has had to monensin or the amount of damage done.
Here are a few links that may help:
Studies on Horses and other animals
The most important thing to prevent Monensin poisoning is to keep horses away from cattle feed, and to make sure that the feed company that you purchase your horse feed from does not manufacture medicated cattle feed or does not make their horse feed in the same place that they make their cattle feed.