How do I know what carrier my friends iphone 4 was made for?

I am planning on buying my friends iphone 4. I have AT%26amp;T and I don't know what the iphone is. I would not like a "jailbroken" one because of the bugs and other problems so I want to make sure that the phone is AT%26amp;T. How can I tell?How do I know what carrier my friends iphone 4 was made for?The AT%26amp;T version would have a SIM tray on the right. Jailbroken iPhones don't have bugs btw.How do I know what carrier my friends iphone 4 was made for?Ask your friend.