How to find if a function is inbuilt function or user-defined-Collection of common programming errors

One, perhaps naive, approach would be to test whether the function name exists a property of the document:

function newFunction (){
    return true;
}

console.log('newFunction' in document, 'toString' in document);

This is, of course, not exhaustively tested and does fail if the function is created as an extension of a prototype, for example:

function newFunction (){
    return true;
}

Object.prototype.newFunctionName = function () {
    return 10 * 2;
};

console.log('newFunction' in document, 'toString' in document, 'newFunctionName' in document); // false, true, true

JS Fiddle demo.

Given that it also fails for 'eval' in document (in that it returns false), then this will, or can, only work to identify prototype-methods of the Objects. Which is, at best, an incomplete solution.