How to tell if an iPhone 4 is Sprint, Verizon, At&t, or Tmobile?

I'm about to buy an iPhone 4 from some guy off of Craigslist and I was just wondering if there's any way to know if the iPhone is sprint, since that's what company I use.

Should have the providers logo on the back. Or you can turn on the phone and while it loads up itll say what provider it is