Obama says if he tried, the florist might not believe him. Tyler Cowen disagrees:
What could you say to prove, over the phone, that you are the President of the United States? If you assume the florist is at a working computer terminal and can access Google, you could promise to answer questions about your life, and to answer them so quickly the florist would not think you are googling to those answers. Plus you are dialing from a 202 area code and you sound like President Obama (because you are President Obama), whose voice is well-known and distinct. I would think he would have an especially easy time establishing his identity over the phone.
Furthermore the audience, wondering that maybe you are the President of the United States, would fall into the deference mode, even if some residual doubt remained.
Impersonating the President of the United States might draw interest from the law, or at least an inquiry, and that would discourage potential pranksters and make your claim more credible.
Who would have a tougher time establishing a credible identity over the telephone? How about Lady Gaga?
The person to ask would be the butcher from whom she bought her meat dress.