Is my iPhone 4S really unlocked?

So, I have an iPhone 4S that I want to sell online. I want to sell it as unlocked, but I want to make sure that it is, first. I'm looking at all of the messages on this forum as well as in other places. The two things I do not have are the following:

1. A 4S-size SIM I can call with.

2. The "Congratulations" message on iTunes. I know this used to happen, but I haven't tried it since iTunes 10 and iOS 6. Does it still happen with iTunes 11 and iOS 7? Futhermore, this page doesn't seem to mention anything about "Congratulations".

What I do have is the following:

1. A phone that I can restore to 7.0.2 through iTunes and activate with a Telcel (Mexico) SIM.

2. A senior Applecare advisor who says that my phone's serial number is listed as Unlocked.

I can't make calls because the SIM gets deactivated after a few months of non-use and it's been about 11 months since I used it. But iTunes downloads the carrier settings and everything works fine, I just can't test the phone as a phone since the SIM doesn't have credit. Wouldn't I have hit a bump in the activation process if the phone was still carrier locked?

If you can, please reference a source.

Many thanks!!