Not 100% sure what you're trying to do, but if you're trying to get the result of calling a setter (to see what, of any, kind of error occurred), you can:
1) use events or signals to indicate a problem:
foo.addEventListener(MyEvents.APP_ERROR, this._handleError);
foo.setter = bar;
2) change the setter to be a function with a return value:
public function setFoo( bar:Bar ):ErrorType;
3) make use of a lastError
property that gets set when there's a problem:
foo.setter = bar;
if( foo.lastError != ErrorType.NONE )
// do something