No.. You sort the wheat from the chaff... And I think you are quite wrong... The police have to decide who they think is telling the truth based on the evidence.. Based on your thinking no case, would ever progress... Do we know Ben Needhams grandmother is telling the truth.. No independent verification
I'm sure the police form opinions just like everyone else. What they should avoid is allowing their opinions to become firm beliefs. The first investigators didn't believe what they were told about when MBM was last seen, for example. They thought 17:30 was independently verified by Cat Baker. The evidence given by DP and GMc relating to 18:40 and 21:05 was qualified by 'if'.
By contrast we have OG making assumptions. Simon Foy said:
"It was perfectly clear to us that the McCanns themselves had nothing at all to do with the actual disappearance....It was just obvious..from..you know..that everything stacked up that they were, you know, they were where they were when the child went missing"
Podcast; Gomcalo Amaral 23:30
In my opinion that is a nonsensiclal statement. Foy is assuming that he knew the time at which MBM went missing. He didn't.
FYI; I have no intention of commenting on the Needhan case.