I want to buy a iphone 5 from someone, but how do I know if its unlocked?

I want to buy it off someone but how do they unlock it? The lady thats selling it said she bought a iphone 6 and is waiting for that one in the mail. Once she gets it she said she's selling her phone. Should I buy her i phone 5? What does it mean to be unlocked?

Unlocked means it is able to work with more than one carrier. A way to verify this is to have 2 SIMs from 2 different carriers to see if the phone picks up both.