1. Preview the question in your web browser
2. Right click over the form field and select "Inspect Element" from the short-cut menu
3. The Inspect Element browser will appear, and you'll see the markup of the page -- including a value like id="...." -- as shown below. This is the element you're looking for.
4. If you are trying to styalize this element, you can click the + sign in the right most pane (to the right of the "Filter Styles" search box. From there, you can create your custom styling, and then copy the rule to your template.css file.