How to Know Which HTML Element is Causing Vertical Scroll Bar

Question

I am learning Bootstrap. On the getting started menu, there are few templates given. I am playing with that. One example is about fixed Footer. I used nav from previous example

Answer 1

Ok, I got that if I change from margin-top to padding-top for container to adjust for nav then problem is solved. I reached to the solution after deleting elements in Firebug. So, quick fix problem is solved but my questions are still open.

How to know which element is causing scroll bar? Any trick?

Also, why margin-top is not working but padding-top has worked?

To make clear where I made change, I am adding the modified CSS:

   /* Adjust Nav */
    #wrap > .container {
        padding-top: 60px;

    }