I guess you could jam a hot dog in there, drive around on a cold day. If it gets cut in two you know the shutters are working.It's kind of like the light in the refrigerator, its hard to prove or know that it goes out when you close the door, but you gotta have faith that it does! lol
I assume mine work as designed, but I honestly haven't seen or heard them move. I do recall some talk last winter where someone had problems with them freezing up while driving in some heavy wet snow. But for the most part there has not been much discussion about them.