Conversation so far:
When was J.
Paul Getty's grandson kidnapped?
July 10, 1973.
What did the kidnappers want?
demanded by telephone a $17 million payment (approximately $93.7 million in 2017.
Why was he the target?
"the family suspected a ploy by the rebellious teenager to extract money from his miserly grandfather..
how old was his grandson?
16-year-old.

Question: How did they know he was really kidnapped or not?
Self-contained rewrite: How did the family know Paul Getty's grandson was really kidnapped or not?