Suppose I had perfect knowledge about the economy. So I knew exactly what the right monetary policy would be to keep the economy growing along some sustainable path. And I knew exactly what that sustainable path would look like.
I would then be able to tell the difference between:
1. Monetary policy is too loose, and the economy is in an unsustainable boom. Maybe the boom can be prolonged for a bit, by making monetary policy even looser. But eventually the boom must end, and ouput and employment will fall. Maybe they will even fall temporarily below the sustainable path, because adjustment is difficult. Either way, the end of a boom will look like a recession.
2. Monetary policy is not too loose, and the economy is growing along the sustainable path. Then monetary policy suddenly becomes too tight, and output and employment drop below the sustainable path. The economy is in recession.