They can't. That's why we have to say that you cannot be compelled to produce a password, because the alternative is that you go to jail for forgetting.

> Following the forensic examination, the Government moved to show cause why Rawls should not be held in contempt for his failure to comply with the Decryption Order. Two hearings were held on the issue in which, “Rawls offered no on-the-record explanation for his present failure to comply.” Based on the evidence presented, the District Court found that Rawls remembered the passwords needed to decrypt the hard drives but chose not to reveal them because of the devices’ contents.

The legal process may not satisfy your epistemological requirements, but it allows courts to make these findings. The Fifth Amendment has nothing to do with “going to jail for forgetting.”