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

Arrow function of JavaScript does not have prototype property.

function isArrowFunc(fn){
    return typeof(fn)=="function" && fn.prototype === undefined && !(/\{\s*\[native code\]\s*\}/).test(fn.toString());
}

isArrowFunc(()=>{}) // true
isArrowFunc(function(){}) // false
isArrowFunc(123) // false