Common problems with a 1991 yamaha xt 600?

I am looking at a 1991 yamaha xt 600 dual sport tomorrow and wanted to see what the common problems are with them. The guy says its not running but was running good before. He said his brother took it out then when he came back it wouldnt start. Im hoping to get more info about what happened tomorrow but I just wanted to see what common causes would make it not start. I am somewhat mechanically inclined so i know the basic stuff like spark plug, fuel, make sure it has spark but I dont know much past that for motorcycles (more car/truck guy) Thanks for the help!
Ok, Yea my friend has a 1985 xt 600 and we are going to try his battery and see
does anyone know a simple way to tell if the CDI could be bad? Its not my bike so I cant take it anywhere or take it apart really