JavaScript ES6: Test for arrow function, built-in function, regular function?

Question

Is there an elegant way to tell Harmony\'s slim arrow functions apart from regular functions and built-in functions?

The Harmony wiki states that:

Answer 1

• Convert Function to string toString
• removing all spaces of this string.

• if ")=>" exists with index greater or equal to 1 => 99.99% is an arrow function .

      F.toString().replace(/\s+/g, '').indexOf(')=>')>=1
  

DEMO :

var fn1=function(e){
   e.target.value=new Date();
};

var fn2=(a,b)=> a+b;

function isArrow(name,F){
         if(F.toString().replace(/\s+/g, '').indexOf(')=>')>=1){
                 console.log(`${name} is arrow-function`);
         }else{
           console.log(`${name} is classic-function`);
          }
}

isArrow('fn1',fn1);
isArrow('fn2',fn2);