I've decided that the only way to be sure your horses are getting cared for the way you want is to care for them yourself, which, sadly, isn't always possible.
Occasionally I will be out riding during feeding time, and twice in the past month or two I've come back after feeding to find my horse's corner feeder empty, but that day's feed gone. There's one horse on that aisle whose owner comes to feed him herself, and I suspect he's occasionally getting the feed intended for the next horse down, while my horse, who is at the end of the aisle, is getting left with nothing.
To make things worse, his supplements are in his evening feed, including one for his seasonal allergies. If he doesn't get that one, he starts coughing. I asked the feeder and BO to please make sure it doesn't happen again, but it makes it very hard to trust that he's being fed properly the rest of the time when I'm not there to check!