Personally, I play that spells cast on things are visible, but not to any great extent. You can see that a sword has Bladesharp as it looks sharper, whatever that means. Bludgeon makes weapons look heavier, Truesword makes them look shinier and so on.
However, I don't have targeted spells having a substantial visual effect. So, in my games, you don't see a streak of magic coming from someone and then feel a 1 point of damage to a random body part. Instead, I play that you know when you have been targeted by a spell, but don't necessarily know where the spell has come from.