In practice, most people can conduct a reasonable verification through a series of challenge/response interactions based on shared knowledge, should they need to do so. Mentioning something done, said, or shared in private recently would suffice in many instances.
For more robust tradecraft, should you need it, a set of one-time codes (passwords or passphrases) might substitute.
When the former head of InterPol was arrested in China, he managed to alert his wife through the use of a duress signal, an image of a knife:
Not subtle, but effective.
In the film Capricorn One (1978), one of the astronauts alerted his wife by referring to a holiday they'd recently taken together, by mis-stating the destination as Disneyland, rather than Hollywood -- the land of make believe -- as it had actually been, which led to the revealing of the hoax mission.