That's one of the great things about the movie. If there are any clues, they are very subtle. Even after you have seen it once, it is well worth watching again. One of my favorite movies, that it seems very few have seen.
The whole ending is the fact that Verbal was lying about every little detail. Even the name of the contact. Then, Verbal isn't even really Verbal. As JJ said, you don't know what is really true. You only know the facts, which was that all these men died on a boat.
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"It is difficult to say what is impossible, for the dream of yesterday is the hope of today and the reality of tomorrow." - Robert Goddard