How could I check in JavaScript if the function I am calling is expecting an argument?

There is the documentation for Mocha. There is the following code sample:

it('should take less than 500ms', function (done) {
  this.timeout(500);
  setTimeout(done, 300);
});

Also, the following test is valid:

it('should take less than 500ms', function () {
});

What JavaScript tools does Mocha use to determine if someone will use the done or not?

E.g. if I have a method:

function incrementIfExpectsSomething(callback) {
    if(/*callback has a parameter in its definition*/) {
        //increment
    }
}

How could I implement the /*callback has a parameter in its definition*/ part?

I do not have any applications straight away. I am asking this question just to have a better knowledge of the JavaScript.