Question: How did Sonny know that the black limo driver was a cop?
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.
Answer: The girls who sneaked into the ward had brought alcohol and everyone was partying before McMurphy and Chief were to escape. Everyone got drunk while waiting for Candy and Billy to finish, eventually passing out. Candy and Billy probably just assumed McMurphy and Chief had already left or they'd fallen asleep.
raywest ★