IPhone :: Testing 4 ATT Carrier Unlock With Custom Config File?

I just had my iPhone 4 unlocked by ATT, but how do I test that it is unlocked? The Apple knowledge base article [URL] says that once the iPhone has been unlocked I should get a message in iTunes saying: "Unlock Complete. Congratulations, your iPhone has been unlocked. To set up and sync this iPhone, click Continue."

I did not get that message, but neither did I get an error message. Since ATT's (possible) unlock I have updated the software to 5.1.1, and done a complete backup and restore. I bought the iPhone used from a friend several months ago, and then created a custom config / profile with Apple iPhone Configuration Utility. This allowed me to use the phone as a Pay As You Go phone on ATT's network, and has worked this way with no problems for the last six months. The phone has not been jailbroken, or unlocked with a third party unlocking tool.

In the next few weeks I will be traveling to the UK where I want to be able to use a local number, but want to make sure that this will work before I travel. How do I test ATT's unlock without a Tmobile micro sim that I dont have?

Info:
iPhone 4, iOS 5.1