if you're looking to access the fields using client-side script, which I think is the case, you can view the source of the webpage in the browser. look for the textboxes in the source and find the id attribute of each textbox. you can then reference the textbox in client-side script using the value contained in the id attribute:
var textbox = document.getElementById("ID OF TEXTBOX GOES HERE");
while this is perfectly valid and will work in just about any browser, I'd always favour using a javascript framework such as jquery when I'm looking to interact with the HTML of a web page. provided you have jquery referenced in the page you would then write something like this:
var textbox = $("#ID OF TEXTBOX GOES HERE");
Bear in mind that with jquery, prefixing the id with a "#" is crucial, or it won't find that element by it's id.