How can we tell he's of low moral character? Why, by 18th-century German philosopher Immanuel Kant's categorical imperative, of course.
Act only according to that maxim whereby you can at the same time will that it should become a universal law.Now, this impersonator is acting according to a maxim that might read something like this: "I should impersonate a park patrol officer." It's pretty clear that maxim can't be universalized, or made so that everyone in the world acted according to it. If everyone claimed to be park patrol officers, we wouldn't know who the real park patrol officers were.
The natural order would be disrupted, confusion and chaos would abound, and inevitably our parks would become dens of treachery and sin. Muggings, lootings, and open fornication would abound. This criminal had better shape up, or the full force of German ethical thought is going to come crashing down on him like the riot club of a properly deputized Minneapolis park patrol agent.
Fortunately the scallywag isn't very good at impersonating - he dresses in plain clothes, so it's easy to spot the trickery. Police describe him as between 35 and 50, standing six feet tall, and of "a thin build." If you spot him, they ask that you call 911 immediately.