The detective destroyed her notes after she wrote her report by herself. How does anyone know if she is telling the truth since there is nothing to compare her report to?

Officer notes are not required and unless they were taken inside the crime scene is not even allowed to be called into evidence normally.

But the police report itself is evidence she is telling the truth, notes are not required to be keep nor normally allowed into evidence.
Officers are normally told not to take them inside the crime scene and to destroy them after the final police report is done.

The notes have nothing to do with the truth, they would merely show what was on the report. The officer when they sign the report state that it is true.