Welcome to MR Da5id. I have never purchased a refurb, but if I did of course the first thing I would check would be phyisical condition.
If that was OK, I'd get the battery charged and start loading and listening to music to make sure it loaded and sounded OK.
While listening I would work all of the controls to make sure they operate as they are supposed to.
Lastly, I would run a test on the battery to see that it lasts an appropriate amount of time.
Hope that helps.
