Which Beth is real in Rick and Morty? The clone call happened off-screen
Which Beth is real in Rick and Morty? What the show has actually confirmed about Beth and Space Beth, and what the creators themselves have said about it.
Rick and Morty has done a lot of cruel things to its audience, but nothing quite so cruel as this: the single most important decision in the Beth storyline — did she stay, or did she leave and get replaced by a clone — happened entirely off-screen, years before anyone brought it up again. The show simply cut away and let everyone stew.
A quick refresher, for the Jerrys among us
In series three, Rick offers Beth a deal: go off and find yourself in the infinite cosmos, and a perfect clone will keep your seat warm at home. The episode ends before we see her answer. A few episodes later Beth outright asks him, "Am I the clone?" and Rick, world's most reassuring father, gives her a wobbly little "no" and a speech.
Then the series four finale drops Space Beth on us — scarred, be-mulleted, wanted across the galaxy — and confirms the cloning absolutely happened. The kicker: Rick swapped the labels on the tanks and scrubbed his own memory so that even he doesn't know which daughter is original stock. Both Beths, to their credit, eventually decide they'd rather not know either, which is either radical self-acceptance or the most Sanchez-flavoured avoidance imaginable.

The people responsible are no help whatsoever
Dan Harmon has been asked about this roughly one million times, and his answer at PaleyFest was essentially "the mystery is the point":
"She's Schrodinger's Beth right now."
He's framed the ambiguity as the real reveal — a portrait of Rick's commitment to never committing, and what that does to a family. Deep. Unhelpful. Classic Harmon.
And if you're hoping the cast got a secret briefing, bad news. Sarah Chalke, who voices both Beths, admitted to Inverse:
"I didn't know as we went along — and still don't — who's the clone."
The woman plays both of them. Nobody told her. There is no vault with the answer in it; the answer is that there is no answer, the call happened off-screen, and the show has been dining out on that empty space ever since. Wubba lubba, et cetera.