How do I know if the interpreter did a good job?

Presenters or organizers of events are not going to be aware of how well the interpreter has undertaken the job - usually they are monolingual. Clues can be gleaned however: 

Did questions from the visitors reveal shortcomings in the information? Were the questions relevant? 
Did two or three sentences get interpreted as one or two words and vice versa? 
Was the interpreter taking notes? 
Did the audience talk amongst themselves or appear perplexed? 
Did the interpreter engage in discussions directly with the audience ignoring the presenter?