Can a federal judge after reviewing the record in the process of making a de novo determination, have doubts concerning the credibility findings of a federal magistrate?

Neither the statute [the Federal Magistrates Act] nor its legislative history reveals any specific consideration of the situation where a district judge after reviewing the record in the process of making a de novo "determination" has doubts concerning the credibility findings of the magistrate. The issue is not before us, but we assume it is unlikely that a district judge would reject a magistrate's proposed findings on credibility when those findings are dispositive and substitute the judge's own appraisal; to do so without seeing and hearing the witness or witnesses whose credibility is in question could well give rise to serious questions which we do not reach.

United States v. Raddatz, 447 U.S. 667, 681 n.7 (1980) (emphasis in original); see id. at

Page 406