I would, at a minimum, test the straight stitch and zig-zag stitches.
Also test the reverse stitch, needle up/down feature, stitch length and width features.
Listen as you sew for noises that might indicate that something is wrong with the machine and ask the owner to explain.
Does a bobbin wind properly?
Does the light work?
Does it come with a straight stitch throat plate?
Is there an extension table?
Does it come with a walking foot? If not, how easy is it to get one, particularly with no dealer nearby and the age of the machine?
Can you drop the feed dogs? If so, does the machine come with a darning foot for free motion quilting? Depending on her asking price, availability of these items at purchase may be important to you since there is no dealer where you live.
Does the current owner still have the instruction manual that came with the machine?
Has the owner kept a file of all the service and repair work done throughout the machine's lifetime? If so, what service issues, if any, did it have?