Full Idea
The classic manifestation of being able to think about other individuals' mental states is success on the 'false belief test', which requires attribution of mistaken representations to other agents. Children aged three or four can do this.
Gist of Idea
Young children can see that other individuals sometimes have false beliefs
Source
David Papineau (Thinking about Consciousness [2002], 4.7)
Book Reference
Papineau,David: 'Thinking about Consciousness' [OUP 2004], p.115
A Reaction
There may be an other minds problem, but there is empirical evidence that we can 'read' the minds of others (from their behaviour) even if other animals can't. That seems to be clear, even if folk psychology is fiction, and we make mistakes.