Much of the assessment of the quality of your hay can be done through simple observation. It should have a high leaf content , few weeds, and should smell pleasant and slightly sweet. If you drop a flake of hay from a height of a few feet, you should not see clouds of dust rising from it. Dust is usually an indicator that the grower had the tines on his harvester set too low. To really determine the nutrient content of your hay, you'll need to do a hay analysis.
Appearance is a poor indicator of nutritive value.
Even grass hays that appear very similar can vary in protein content by two to three times. You also might request other data, such as the copper or selenium content. Your feed specialist or extension agent can help you interpret the results and balance the rations in your barn to complement the nutrients in your hay. Hay is the easiest and least expensive way to provide the essential fiber in your horse's diet at a time when he's probably on a reduced work schedule and pasture is likely to be inadequate. Not only that, but eating hay will help keep your horse warm when the wind blows bitter.