version of Andrea's

isNativeFunction.js

var isNativeFunction = function (f) {
  try {
    Function("return " + f.toString());
    return false;
  } catch (_) {
    return true;
  }
};

// this will return true
isNativeFunction(function f(){
  f.toString = function () {
    return "function () {[native code]}";
  };
  return f;
}());

// this as well ...
isNativeFunction({});

so ... as summary, I would suggest this one

var isNativeFunction = function(Function){
  var s, toString = Function.prototype.toString;
  return function isNativeFunction(f) {
    try {
      // object.toString can be reassigned
      // native toString from Function.prototype
      // should be borrowed, sandboxed, to ensure
      // that we are using the right function
      // rather than one redefined by a user
      s = toString.call(f);
      try {
        // native functions cannot
        // be evaluated (as de facto rule)
        Function("return " + s);
      } catch(_) {
        // if evaluation fails
        // we can assume it's native
        return true;
      }
    } catch(_) {
      // if toString.call failed
      // the object has no [[Call]] property
      // then is not a function
    }
    // all cases except the first nested catch
    return false;
  };
}(Function);