Conversation so far:
When was J. Paul Getty's grandson kidnapped?
In Rome on July 10, 1973, 'Ndrangheta kidnappers abducted J. Paul Getty's 16-year-old grandson, John Paul Getty III, and demanded a $17 million payment for his safe return.
What did the kidnappers of J. Paul Getty's grandson, John Paul Getty III, want from J. Paul Getty?
In Rome on July 10, 1973, 'Ndrangheta kidnappers abducted J. Paul Getty's 16-year-old grandson, John Paul Getty III, and demanded a $17 million payment for his safe return.
Why was J. Paul Getty's grandson, John Paul Getty III, the target?
When Italian gangsters abducted the grandson of J. Paul Getty, the richest man in the world, they thought they’d seized an easy fortune.
How old was J. Paul Getty's grandson, John Paul Getty III, when he got kidnapped?
In Rome on July 10, 1973, 'Ndrangheta kidnappers abducted J. Paul Getty's 16-year-old grandson, John Paul Getty III, and demanded a $17 million payment for his safe return.

Question: How did they know he was really kidnapped or not?
Self-contained rewrite: How did J. Paul Getty and family know J. Paul Getty's grandson, John Paul Getty, III, was really kidnapped or not?