Delphi

The sanctuary of Apollo on the slope of Parnassus, and the oracle every Greek state consulted before doing anything serious.

Herodotus tests it. He records Croesus sending to seven oracles at once with a question only he could answer, to find out which were genuine, and reports that Delphi alone got it right. He then records Croesus misreading its answer about the great empire he would destroy, which turned out to be his own.

Before Salamis the Athenians were told to trust in the wooden wall. Herodotus gives the argument that followed over what a wooden wall was, and does not tell the reader who was right until the event does.