Just looking at your horse can give you a lot of information about his health, but performing an accurate body condition score is still the best way to monitor his weight over the long term. An overweight horse is more likely to suffer from joint, cardiovascular, metabolic and respiratory illnesses. An underweight horse on the other hand will struggle to build muscle due to the lack of calories, and may in fact lack energy generally. A horse being underweight can also be a sign that he is unwell or in pain.
As an additional point to note, a shiny coat is also an indicator of good health. If there are health problems or your horse is being underfed his coat may look dull.