Sonny found out where the meeting would be by calling his contact at McCluskey's precinct.

The novel provides further detail:

Sonny's contact at McCluskey's precinct was Detective Phillips, who tried to protect Michael outside the hospital. He's the guy Sonny contacted in the movie. A police captain had to leave word where he'd be at all times, which was how Phillips knew where he'd be.

In the novel, Sollozzo had a man in the restaurant at another table, to whom he looked when Michael asked to go to the john. The man nodded as if he'd checked the bathroom. But, after Michael whacked Sol and Mac, he pointed the gun at the man at the table, who looked away and showed his empty hands. Novel also says the police questioned Sol's driver, who claimed he didn't know anything.