How would you ascertain that police officers have acted appropriately in obtaining a confession?

The single best way to ensure officers have acted correctly is to pass a law requiring police to record all interrogations.
Recording interrogations is generally beneficial because it means that police can track contradictions in a suspects story and review word-for-word what the suspect said without having to rely on fallible memory. However, recordings also substantially reduce false confessions, both by making it harder for police to use improper interrogation techniques and by making it more obvious when a confession has occurred under duress or due to a civil rights violation.
For example, a false conviction in Texas was caused when police officers refused to give the suspect access to a lawyer, a right he was guaranteed by the Constitution, and then informed him that confessing was the only way to avoid a death sentence. They continued this abuse over 12 hours until he confessed.
Mandated recording would have plainly shown this egregious violation of the law by the police. But even if they hadn't done so--even if the suspect had never asked for a lawyer--a video of the constant pressure and threats of death would provide strong evidence to judge and jury that the confession was not reliable.
https://www.innocenceproject.org/how-to-stop-false-confessions/