The Sending spell says:
You can send the message across any distance and even to other planes of existence, but if the target is on a different plane than you, there is a 5 percent chance that the message doesn't arrive.
If the spell fails when trying to communicate with someone in another plane, does the caster know that the spell failed? Or is it indistinguishable from the spell succeeding and the target choosing not to respond?
Generally speaking, the rules are silent on whether spellcasters know details about spell failure. However, in this case, I think that the wording specifies that the caster cannot know. The spell text draws a distinction between the sending and the arrival of the message; the caster is involved only in the actual sending of the message and has no control over its arrival.
It seems most likely that the caster would not automatically know if Sending failed or was received.
However, there is a clause that
The creature hears the message in its mind, recognizes you as the sender if it knows you, and can answer in a like manner immediately.
If the caster is sending a message to someone they know and they do not receive a response back, in most cases it would be safe to assume that the sending failed.
While it's definitely up to interpretation, I would personally rule that the caster does not know. After all, the intent seems to be that the message "gets lost in transmission", in which case it would cease to be perceivable by either party.
Fortunately, there seems to be a solid work-around: just make sure to ask the other party to respond in your message. ...unless you want to rule that the return message has its own 5% failure chance, in which case, as The Two General's Problem will tell you, you can never be 100% sure it worked no matter how many messages you send!
