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Are Hunter's Mark / Hex / other spells like that noticeable to the target of the spell?
I've watched Critical Role, and it looks like the only thing the character needs to do is pass a sleight of hand check if they are visible to the target (but Hunter's Mark has a verbal component, not a somatic one?).
The basic question is: Can the target tell in any way if they are now marked, by way of seeing, hearing, or just feeling like, "Oh man, I feel magic on me." Would it make a difference if the target was a magic user, or a commoner, or a beast?