How can you tell? Unless you are politically motivated to exaggerate, then all we really know is that there have been some high profile instances where police reports have been falsified. But I don't know how we can reliably expand that to all police reports.

Fundamentally, police officers are sworn, and so their word carries more weight than a non-sworn citizen. This is sensible, but it ought to come with strings attached. Lying, for example, should reliably results in a perjury charge.