The Fae have developed very potent Glamour magic to where every creature is born with it some don't know till a certain age while other's know off the bat. When a Fae shifts into their true form it's called a Shimmer and usually normal people can't see it when it happens but Hunter's can.

There are those occasions when a Fae can't control their shimmer due to either aggression or some other form of emotion and that's when a normal person can indeed see The Shimmer and a true form of a Fae.