Hi Derek, watching the weather forecast, changes in the temperature and checking to see if your horse is comfortable under their current rugs will let you know if they need a lighter rug.
Also look at yourself, if you are going out in all your extra winter clothes that you have been wearing all winter and within 10 minutes of being with your horse you start to remove the layers then also check your horse. If you are too warm under your normal winter coats chances are your horse is warm under their winter rugs!
Pop your hand in under your horse's rug at the shoulder. If they are toasty they will be fine, if it feels hot and sticky or sweaty then they need a lighter rug.
Checking their ears also is a good indicator. If their ears are cold they are cold. If the ears are warm then they are warm and happy.