Just as it doesn't have their caliber of data, macroeconomics also lacks the kind of scientific culture enjoyed by biology and chemistry. In the hard sciences, models are built to explain data; that's their only purpose. But in econ, models are often used simply as storytelling devices to explain an idea about how the world might work.
The best economists are well aware of their ignorance. During his recent graduation speech at Princeton, Federal Reserve Chairman Ben Bernanke half-joked to the crowd that "Economics is a highly sophisticated field of thought that is superb at explaining to policymakers precisely why the choices they made in the past were wrong. About the future, not so much." Greg Mankiw, one of the world's most famous macroeconomists (and my PhD advisor's PhD advisor) put the sentiment this way in a 2011 New York Times column:
"After more than a quarter-century as a professional economist, I have a confession to make: There is a lot I don't know about the economy. Indeed, the area of economics where I have devoted most of my energy and attention -- the ups and downs of the business cycle -- is where I find myself most often confronting important questions without obvious answers..."
What all this means is that when an economist tells you something that is based on a theory or a model, you should be very, very skeptical. And the more complicated the theory or model is, the more you should be suspicious. For example, in a recent Wall Street Journal column, Stanford economist John Taylor called for fiscal austerity, and justified his recommendation by saying it came from a "modern macroeconomic model." I looked through that model and found a lot of assumptions that a lot of other economists would disagree with. But the average Wall Street Journal reader would have no way of knowing that. So beware of economists bearing fancy models.
If economists ever do succeed in developing formal models that work better, then we'll be able to go to them with questions (like "Should the Fed print more money?") and simply trust their expert advice. But until that day, all economists can really give us is intuition, suggestions, and ideas. Like the Royal Physician, each of us then has to decide for him/herself what we think is the best medicine.
So when you listen to economists, the key is to try to understand why they think what they think. For example, Paul Krugman thinks that monetary policy doesn't work well in a depression, because nominal interest rates can't go below zero, and because the Fed is not always good at convincing people that it will allow inflation in the future. Robert Barro thinks that fiscal policy doesn't work, because people anticipate the future taxes needed to pay for today's stimulus, and reduce their consumption today in order to save up to pay those future taxes. Most people can understand these basic ideas, and decide for themselves which they think are plausible, and which they think are unrealistic.