[dymk]

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.