Dog Day Afternoon

Question: How did Sonny know that the black limo driver was a cop?

Chosen answer: He didn't. He just assumed the cops would try something like that, substituting a cop for the real driver. What he didn't anticpate is that they had substituted cops for everyone around the bus when it arrived, and when he replaced the black driver he would have to (unknowingly) pick a cop, regardless. Incidentally, this part of the story is completely factual, it actually happened that way.