How would you check an Iphone if you buy from a person?

I'm thinking about buying this Iphone 5s on Craigslist from this person. I give her cash and she gives me her phone. So what would I need to check to make sure the phone is in good condition and working properly? I don't mind a couple of scratches here and there but I want to make sure that phone is working.

Pull Out Da Heater #Gratata! And You Get A Free Phone

You should get a new phone, with a warranty, if possible!