You have to look carefully at your grass to ensure that there are enough different varieties to ensure that the horse gets all the vitamins that are needed - each type of grass provides different vitamins. If you have poor grazing then your horse may require a balancer, but our yard (and our local riding school) have really good grazing on pasture which is well managed. The type of hay/haylege that you feed also needs to be examined to ensure that it is cut from good quality grass. Most horses are fed additional hay throughout the winter in some quantity and this is what makes up their vitamins when the grass is at poorer quality in the depths of winter.
I do however think that the best way to tell if your horse requires any additional feed is to assess their health. If you have a horse with good feet, bright eyes, a good weight and nice shine to their coat - then they are probably getting everything that they need already. If not, then you may need to feed additional supplements as necessary.
It's a bit like giving vitamin tablets to children. If they have a well balanced diet and are fit and healthy then most do not need to have them. It probably won't do them any harm to have extra (although this is debatable) but they won't get any extra benefit from them.