Reply to Buying a used S4 what should I look for?

Buying a used S4 from someone around my area. Does anyone have a checklist or can help me out with what I should check for when I meet up to buy the s4?

I don't have traktor pro and neither does the guy selling it to me, so I don't know how I can check to make everything works before I buy it... I don't want to buy it then find out 2 hours later that some knobs or sliders dont work correctly.

Any suggestions would be helpful, thanks!