I'm going to exchange my iphone 4s gevey with experia t?

I want to know that what should i check before exchanging my phone that experia t has mo fault.is there any numbers to dial and check it sensors and all like samsung have. I found the guy on olx and tomorrow i'm going to meet him. What should i do and what are the most scams and fault on this type of phone while buying it used from olx or ebay.

Do research online and see what a good condition one should be like