How could we ever check?

Network traffic logs.

Your voice is always sent to Apple to process it since Siri doesn't work locally; what we are wondering is whether that voice recording is permanently stored or destroyed as soon as it's processed.

So… This is just a “please, don’t record what I send you”? How could anyone believe apple will obey this demand?

How can anybody trust anyone to do anything?

They have a good track record of respecting user privacy.

But.... Let's try again... How do we know that?

You don't know it for a fact. You trust them to not do it.

Yes, let's try again, how do you know you can trust anybody to do anything?

Do you hold a gun to their head? Do you trust the courts to uphold written agreements/contracts that dictate the other party's behavior? Do you observe their past behavior and use that to guess future behavior?

In 99% of our daily actions, we use the last option, and right now is no exception.