look if touch-id works - if the home button has been replaced as part of a screen repair, it won't. (that doesn't mean, that the screen hasn't been replaced with the old home button intact, though). and check the functionality of all physical features, if possible: led-flash, vibration, buttons, headphone jack, lighting connector, speaker, microphone, cameras. the 5s' aluminium is much softer than the 4/4s steel bands, so it's easy to get scratches and dents - so that might give you an indication of how much the previous user cared for it. on the other hand - if it works, it works & a soft frame might even be a better protection from some drops.
you might have to put in a new battery, as a used 5s might be already 2 years old. i'd consider sending it to apple for that - it's not that much more expensive than doing it on your own (especially if you factor in the time it takes you), and those phones are pretty easy to destroy, when opened if you're not careful. take extra caution when opening it, that you don't seperate the two halves too far (no more than about 0,5-1cm) before unplugging the home button - or you'll rip the cord and have to send it in to apple anyway for a new button with working touch-id.
