How do you know if an iPhone 4S is factory unlocked?

Dear Apple community,

I've browsed around the forums, but haven't really found one posting that answered my question. I'm not an international traveler or anything of that sort, and would like to use the iPhone 4S factory unlocked (because I don't want to be hassled with the jailbreaking and software nonsense) with T-Mobile in the US. However, since most Apple stores do not currently have them in stock, I'm planning to buy it off a third-party to save a little moeny. I.E Local people around my area that are selling their 4S for various reasons. If the iPhone 4S is completely sealed and they claim it's factory unlocked, how would I be able to fully know that it is? I'm assuming I would have to open it to find out if it is really factory unlocked (Please clarify!). But if the seller assures that it is and I'm 100% sure that I'm going through with the transaction, I should be able to run it through some quick tests after opening it and before handing over the money.

But long story short: How would I be able to tell if an iPhone 4S is factory unlocked? I'm intending to use it strictly in the US (where I live) if that helps. And please, I know it's more convenient to buy it directly from the Apple store, but it wouldn't hurt to save a few bucks buying it from someone who doesn't have need for it.

Thank you and happy new years everyone!