How did William know there was one camel in the circus?

William's knowledge of the presence of one camel in the circus stems from his previous experience of having seen it there before. This firsthand observation provides him with direct evidence that the camel is part of the circus. Unlike information obtained from external sources such as newspapers or friends, personal experience is a more reliable basis for certainty regarding the camel's presence. It suggests a familiarity with the circus that only direct observation can confer, reinforcing his confidence in knowing what to expect at the event.