I have an unwritten, mostly unclear rule that magic is not something one can cast stealthy or in a low-key way. Magic is always presented as something that is uncommon and outside the normal order of things. I always ask the magic users in our groups to tell us the the hows of casting spells and what it is it that gives away a spell is about to be cast.
I was thinking about codifying the rule into some specifics. My first thought was that if magic is an unnatural occurrence in the world, that the mortal races have a sixth sense when magic is being manifested in an area around them.
Any ideas are welcomed.