What’s the best way to unit test an event being emitted in Nodejs?

If you can guarantee that the event should fire within a certain amount of time, then simply set a timeout.

it('should emit an some_event', function(done){
  this.timeout(1000); //timeout with an error if done() isn't called within one second

  myObj.on('some_event',function(){
    // perform any other assertions you want here
    done();
  });

  // execute some code which should trigger 'some_event' on myObj
});

If you can’t guarantee when the event will fire, then it might not be a good candidate for unit testing.