After watching too many Police investigation movies I came up with a profile for your Litter-Denouncer.
"He's over 50 years old, doesn't have much to do so he sits around, watching out his window and waits for something to happen he could report to the police, because that makes him feel important. Or he could be standing around the fields, if that's closer to the road.
He might even be armed with binoculars, to get a better view, and obviously a mobile phone in which he has the number of the local policestation saved.
He probably knows everybody in the whole area and everybody who's not from around there is suspicous."
So based on that, just mow down any old geezer standing around in the area that you supposedly littered. Just to be on the save side.