"Never trust an economist, until you've checked his math".
True indeed. If you can understand his math, of course - otherwise you have to get someone else (whom you trust for good reasons) to do it for you. IMHO there ought to be a profession that entails nothing but checking the correctness of other people's math AND the correctness of their mathematical modelling.
Therein lies the even greater problem with economics. The math may even be entirely correct - but how can we tell if it corresponds 1-for-1 to any phenomena in the real world? ("Mathematics may be defined as the subject in which we never know what we are talking about, nor whether what we are saying is true". - Bertrand Russell). Worse still, an economist may have modelled some aspect of reality in a reasonably accurate way, and got the math right - but the piece of the real world he modelled wasn't big enough to tell us anything meaningful, useful, or complete.
Towards the end of his long and phenomenally productive life, Sir Isaac Newton confessed that, "...to myself I seem to have been only a boy playing on the seashore, and diverting myself in now and then finding a smoother pebble or prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me". It takes a very great man to say something so modest. Ironically, his words apply in far greater measure to modern economists - none of whom would ever dream of making such an admission.