500
"The first indication that the USSR might be lying about sending nuclear arms to Cuba came on September 18 when, off the coast of Tunis in the Mediterranean, a U.S. Navy frigate confronts a Russian merchant ship and inquired about its cargo. The ship reported that she was carrying only agricultural machinery. Binoculars aboard the frigate indicated otherwise, as the deck was covered with large crates of irregular sizes, which appeared to be the kind that carried disassembled military aircraft."
How did the USA know the USSR was lying about sending missiles to Cuba?