Charismatic mammals remain a linchpin in attracting publicity and funds for the conservation of native habitats and organisms. Unfortunately, the same animals are frequently scarce and difficult to survey. For many, confirming their presence through faecal surveys is the only cost-effective approach. Here we show that, contrary to received opinion, expert naturalists fail reliably to distinguish pine marten Martes martes faeces (‘scats’) from those of foxes Vulpes vulpes. Moreover, their judgement fails completely when the animals and their scats are at their most scarce. This unexpected result from such a well-studied species has important implications for the monitoring of endangered mammals. We recommend that in the future, a multi-evidence approach should be adopted to monitor elusive mammals, involving DNA methods, cast hair identification, camera traps, and non-leading ‘sighting’ questionaires. For national surveys, it may soon become cost-effective to screen large numbers of samples using microarray technology.