What is the test for admissibility of evidence at a preliminary hearing?

'. . .: the rules of evidence normally applicable in criminal trials do not operate with full force at hearings before the judge to determine the admissibility of evidence.' Citing Brinegar v. United States, 338 U.S. 160, 173, 174, 69 S.Ct. 1302, 93 L.Ed. 1879 (1949). And in Note 9, 94 S.Ct. at 994, reference is made to the difference between admissibility of evidence on the merits as contrasted to use of evidence '. . . for the purpose of determining a preliminary question of admissibility . . ..'