How to check if the phone has been paid off in full before buying?

I want to buy a T-Mobile branded Samsung Galaxy Note 3 from someone who claims only been using it for a "month". I want to ask this seller a few questions such as if the seller says the phone has been paid off in full or is unlocked, is there a way to check?

I know T-Mobile offers $0 down payment on phones so I want to be "safe" and not get scammed. Can I request the seller to show paper work or proof that the phone has been paid off in full or the balance of the "EIF"?

Maybe I can ask the seller to do this?? will I be safe??

Pay your T-Mobile bill online

EIP payments and balance information

To make an EIP payment or to check your EIP balance, follow these steps:

1. Log in to My T-Mobile.
2. Click BILLING.
3. Under Equipment Installment Plan (EIP), you can see YOUR EIP BALANCE as well as click View Details to see more information.
4. Click Make a payment and follow the instructions if you'd like to make an EIP payment.