Although, I'm thinking a good HR for both situations might just be "line of sight." If a wizard can see a spell he's casting affect something, he knows if it worked - and if not, why not. If he cannot see it, he can't tell either if it worked or why it did/didn't. This makes for a simple rule that is easy to remember and seems pretty fair - and quite frankly reflects the way I've usually done it anyway

(and that means knowing if
sending works is conditional on where the recipient is - either in line of sight or not).