How to check if an iPhone is factory unlocked

Hi all,

Assuming all goes well, I am getting an iPhone 4S off of Craigslist in the next couple days. I would like to check that it is a factory unlocked model before I go through with the deal, and was wondering if you guys knew how to tell. I read about going to to check, but I wasn't sure how reliable that was... Does anyone have any experience with this?

Thanks in advance!