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

As far as I can tell this should work:

All non-arrow functions when converted to String START with 'function '. Arrow functions don't.

Trying to test for the presence of '=>' is not a reliable way to test whether the function is an arrow or not because any non-arrow function can contain arrow-functions inside them and thus the '=>' can be present in their source-code.