Spur Route 45 in Waldron is clearly a state-maintained route. This marker at the route's endpoint (looking northbound) is a clear indication of that. But there are no markers indicating the actual route designation. You apparently have to retrieve that designation from the official MoDOT map of Platte County.
There's also a marker on the other side of the (very narrow) road marking the end of maintenance in the other direction.
This photo was taken looking northbound on unmarked Spur 45 in September 2008.