What indicators are there that the messages are coming from a superior culture?

In Chapter Thirteen, Petra began to receive thought-shapes from a distant source, but none of the others could sense them. It was difficult for Petra to communicate since she was still so young, but with the help of the others, they puzzled out the woman’s message. She was from a far-off island called Sealand, although Petra insisted the name started with a Z. The woman said Petra’s powers were the strongest she ever encountered. The children were surprised when the woman said they were savages.