Which Carrier and Can I Switch to Verizon

I "inherited" two iPhones from my daughter. One is an iPhone 3 and the other is an iPhone 4.



She doesn't remember whether she used them when she was on ATT or Verizon.



(1) How can I tell



(2) If they were ATT can I change one to Verizon?



Thanks in Advance.



Pete

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