As a horse owner it is important that you take time to objectively assess your horse's weight on a regular basis.
Age, weather conditions, amount of turnout, amount of pasture, illness, and work load can all cause your horse's weight to fluctuate throughout the year. So it is important that you take time to assess your feeding regiment on a regular basis.
I recommend objectively evaluating your horse's weight every 3-4 weeks. This is enough time for your horse to start showing changes in either direction, but frequent enough for your to address these changes in timely manner.
You can do this by tracking your horse's weight using a weight tape or you can take regular pictures with your phone and use them as a comparison. If you are lucky enough to ride/work with friends and family often I highly recommend asking them to help you keep an eye on your horse's condition and in turn doing the same for them.