Almost all the public versions of the scripts can be found here:

and they have several articles:

http://starwarz.com/starkiller/2010/03/the-connoisseurs-guide-to-the-scripts-of-the-star-wars-saga/

With passages like these which document the Katz/Huyck contributions:

March 15, 1976.
The Adventures of Luke Starkiller as taken from “The Journal of the Whills” (Saga 1) Star Wars.
Revised Fourth Draft Screenplay. 156 pages. 252 scenes.
With circa 30 percent dialogue additions by Willard Huyck and Gloria Katz. (The actual shooting script: Ben Kenobi dies.)
Source: Bouzereau: p. 3; Pollock: p. 20; Flynn: p. 13.

SW.SE. 1997.
Star Wars. Episode IV: A New Hope (Special Edition).
Public Version of Fourth Draft with Special Edition changes. No scene numbers. No dates. Annotated by Laurent Bouzereau, including interview snippets from George Lucas etc. Asterisks (*) indicate the most significant dialogue contributions from Willard Huyck and Gloria Katz for the revised Fourth Draft. Printed in Laurent Bouzereau, Star Wars: The Annotated Screenplays, Del Rey 1997.

and

http://starwarz.com/starkiller/2010/05/the-development-of-star-wars-as-seen-through-the-scripts-by-george-lucas/

Lucas had consulted his co-writers from American Graffiti (Willard Huyck and Gloria Katz) to sharpen the dialogue, which he felt lacked humour and bounce, and although he rejected most of what they came up with, their new ideas gave Lucas renewed confidence in his work

From what I remember the 'Star Wars - The Annotated Screenplays' by Laurent Bouzereau, is the place to start. They notate each Katz/Huyck dialog with a '*'

The book is in google.books so you can search through it from here:

http://books.google.com/books?id=eQ7ct2r14w0C&q=%22star+wars%22+%22annotated+screenplays%22+katz&dq=%22star+wars%22+%22annotated+screenplays%22+katz&hl=en&ei=d5I3TZaJK4S8lQf0x731Bg&sa=X&oi=book_result&ct=result&resnum=1&ved=0CCoQ6AEwAA

*EDIT*

Poking through my copy the first significant contribution I find is the Luke/3po bit:

http://books.google.com/books?ei=d5I3TZaJK4S8lQf0x731Bg&ct=result&id=eQ7ct2r14w0C&dq=%22star+wars%22+%22annotated+screenplays%22+katz&q=alter+time#search_anchor

LUKE
Well, not unless you can alter
time, speed up the harvest, or
teleport me off this rock!

THREEPIO
I don’t think so, sir. I’m only
a droid and not very knowledgeable
about such things. Not on this
planet, anyways. As a matter of
fact, I’m not even sure which
planet I’m on.

LUKE
Well, if there’s a bright center
to the universe, you’re on the
planet that it’s farthest from.

THREEPIO
I see, sir.

LUKE
Uh, you can call me Luke.

THREEPIO
I see, sir Luke.

LUKE
(laughing)
Just Luke.

THREEPIO
And I am See-Threepio, human-cyborg
relations, and this is my counterpart,
Artoo-Detoo.

All that is accredited to Katz/Huyck.