I think that this would work better if the person to touch the badge was its' owner. It might even attack if anyone else touched it. My other suggestion is that it might be reasonable to expect someone from the Auror office to be able to cast a Patronus. If the form of an Aurors patronus was known by the public it could be used as a test as Patronuses are unique. The problem with that is that a Patronus can change form.
Maybe it would be a badge, and then if the civilian taps it, it says the Auror's name and rank.