Update: David Bernard figured out why this wasn't working as written.
He writes, "Once you sign back in, you need to go to the account page and click the 'reset warnings' button. I wrote it up on my blog." He tried it and it works now. Thanks David!
App developers often share promo codes with reviewers, forums, etc. Unfortunately, Apple doesn't provide a way to track which codes have been used, resulting in wasted promotions. Fortunately, Kam, the developer of Walkabout, explains how to determine if a code is still available via iTunes.
Kam's procedure is simple but effective. First, he writes, launch iTunes and log out. Next, log back in being careful not to click Save Password. Then, quit and re-open iTunes.
With iTunes running, try to redeem a promo code you'd like to test. If the code has been used, iTunes will display a message saying, "This code has been used -- codes may be used only once." If it's still available, iTunes will prompt you for your password. Just click Cancel and mark that code as still available.
If you try this with your password saved, iTunes will redeem the code. By logging out and opting not to save your password, iTunes will let you know if the code is available without redeeming it.
Well done, Kam! Thanks for sharing your tip.