How does their pasture look? Lots of manure, weeds growing. Basically like its not kept neat, not like a perfect pasture but keeping weeds out, and not a whole lot of manure seen.
Does the land look barren? Hardly any food to eat and just dirt that you see.
Do they have water and shelter? Are they unable to access water and shelter from the pasture that is a sign that they could be not cared for because they should have access to a shelter and water so they could shield themselves from the hot sun and so they dont bake.
What kind of fencing do they have? Barb wire, not a good fence to use ith horses because it can hurt them, but you cannot really report them for that. If their fence looks dilapidated, like its about to fall down then that could be a sign that they are not being taken care of.
How do they look in terms of health? Do their hooves look bad? Cracked hooves or hooves left uncared for to grow long. Are they limping like they are in pain? Sign that they could. Can you see bones like ribs or their hip bone showing? Horses that look skinny or are getting skinnier that could signs of not having proper care and could be another sign of not being cared for.
You may want to report them to the Humane Society or the ASPCA if the signs are bad. If it is just minor there may not be anything you can do.