Check the back of each box. In the USA, there should be no Carrier sticker on the lower center part of the sticker. The AT&T units all have AT&T on the bottom of the big sticker.
In addition, there will be no ICCID numbers on any of smaller sticker on the lower back of the box. This is because the Micro-SIM is not included in the retail packaging of a factory unlocked iPhone 4 packaged for retail in the USA.
Other countries, have slightly different packaging, what country or market are you looking to purchase in?
Just a strong word of caution, there is a big difference between factory unlocked and no-commitment pricing. If it sounds too good to be true that it is often a scam. 😱
Dave