If a child is talented he/she performs well at school. First of all the school papers are the most important references for the teachers to decide if a student is talented or he is not.
Outside the school it is more difficult to determine the child's talent. Like for example in case of W. A. Mozart the opinion of his publicum was important how other people liked his music and one also accepted the opinion of his pesonal big masters, like Bach, of course. Mozart wrote as beautiful melodies as technically worked out music and both aspects was very important at that time.