Using F12 Developer Tools

Modern browsers have an inbuilt tool to aid web developers, which you can open for any web page by hitting the F12 key.

Personally, I have found the Google Chrome developer tool to be the best.

Debugging Javascript
You can debug Javascript buy clicking the Sources tab. You’ll get a list of Javascript files that are available on the current page. You can open any one and click the line number to set a break point. Hit F10 to step through. Don’t click F5 – you’ll reload the whole page

View control layout
You can navigate through the HTML to find a control.

Chrome's Developer Tools in action.

Chrome’s Developer Tools in action.