In Keyser v. Smith (Sept. 2018) the US District Court, E.D. Pennsylvania, articulated many of the factors that must be considered in evaluating the voluntariness of a confession:

The duration and means of interrogation, including whether questioning was repeated, prolonged, or accompanied by physical abuse or threats thereof; the length of the accused's detention prior to the confession, whether the accused was advised of his or her constitutional rights; the attitude exhibited by the police during the interrogation; the accused's physical and psychological state, including whether he or she was injured, ill, drugged, or intoxicated; the conditions attendant to the detention, including whether the accused was deprived of food, drink, sleep, or medical attention; the age, education, and intelligence of the accused; the experience of the accused with law enforcement and the criminal justice system; and any other factors which might serve to drain one�s powers of resistance to suggestion and coercion.