Taste, smell, and apperance. Those are the big ones. If your using a full auto machine(which I have) people get to dependent on the shot time. Yes, it should be in a certain range but you have to practice with your perticular espresso to make sure your actually pulling good shots. There are many guides to how to do this but if your unsure take a beginer espresso class. They will teach you a ton about extraction and timing as well as some of the basic physical mechanics of pulling a good shot.