He should be disected on live TV over this. After the denial, introduce him to a panel of 3-5 body-language interpreters. Have them each provide an analysis of the highly orchestrated maneuver. Then the host can confront him again: "How stupid do you think the American people are to lie about this?"
Unfortunately, opinion is not unanimous on the gesture. Bill O'Reilly had a body language expert, Tonya Reinman (sp?) on his show last night and she didn't think it was anything more than scratching his face, not the bird. That conclusion will give Obama cover for any denial he might make if confronted.