Bart Carroll (hosting): Texas Devin asks a real question for Jeremy:
“May I dismiss my familiar and resummon it to the unseen far side of a
wall or locked door?”
Jeremy Crawford: Wow, okay. So let's, let's take a look at the find
familiar spell.
BC: And this is, yes, Jeremy brought his laptop today so he can look up
things on the fly.
JC: Yeah, normally you know I look up things in the books, but I
realized, why don't I bring in my laptop and I can search things on
D&D Beyond, so BAM I have find familiar here. (Reading) And you can —
as you know, you can dismiss your familiar… and then, as an action,
you can cause it to reappear in any unoccupied space within 30 feet of
you. Strict reading of this? I would allow it to appear in any
unoccupied space within 30 feet of you. It includes beyond a wall and
whatnot. Someone might then, might then ask me, “well Jeremy, what
about the rules on spell targeting that say, hey the target of a
spell, there must be a clear path to it?” It, you know, it can't — the
target can't be behind total cover unless the spell says otherwise.
BC: Yeah.
JC: Here though, we're not talking about the targeting of the spell.
The targeting of the spell is back when you first created the familiar
within its original ten-foot range. This resummoning, all the spell
says is it has to be any unoccupied space within 30 feet of you.
BC: So could I go to the bank, and summon my familiar behind the
vault?
JC: As a DM I would allow it.
BC: A-ah.
JC: But — but,
particularly because I as a DM like to interpret our rules as generously as possible. And especially generously when that's going
to cause the players to get themselves into trouble. Because,
because what you just described, someone might think “wow,
that's broken” and as I, as a DM, I, I think of that — I'm like “ooh
delicious,” because chaos is going to ensue, story time is
coming.
BC: The guards must know this, so what have they placed in the vault
that watches for things teleporting in, and what happens when my
familiar gets absconded with, right?
JC: Right. Well and also, familiars are, are not… particularly resilient,
so there's risk involved.
BC: I would also like to see the familiar just take the money and run.
At what point do they decide to break ranks and just, you know, take
off.
Best Answer
There's no specific rules that cover it explicitly, so we'll have to rely on secondary techniques.
Familiar
The telepathy ability has a specified range - the character can easily go "Hey, are you dead?". No response tells the character the familiar is out of range, or dead.
Two actions allow the character to positively confirm dead-ness. Dismissing the familiar has no range limitation, so the caster can spend an action to dismiss the familiar and a second to recall it. If it doesn't respond to the recall order, deadness is confirmed. (Some DM's may allow it in one action by providing feedback on a "failed" attempt to dismiss.)
Ranger's Companion
You're on the money here - the beastmaster has no magical communication with his companion, and no ability to yank it back to him. He's out of luck, and doesn't have any short-term secondary method that would allow him to determine his animal's status. If the critter doesn't come back, he can spend eight hours and attempt to bond a new one - by strict RAW (the companion has to be dead) if it doesn't work, that tells him the original one is alive.