You can survey the potentially harmful plants in the pasture, but it always seems you can't discover the plant that the horse ate. Analytical procedures for plant agents from animal fluids or excreta are limited. Anything can cause colic if overconsumed. However, we have desirable forages that we plant and try to propagate. Any other plants in your pasture should be suspect.
Pasture management prevents weed poisoning.
A good pasture has an adequate stand of grass, such as bluegrass, orchard grass, or bromegrass, and possibly a legume. It has to be kept fertilized and managed. A good stand of grass will keep the weeds out. If the pasture is overgrazed, remove the horse. Promote desired regrowth with some fertilizer and manage weed control early.