Take fabric and thread (whatever you would normally be sewing on the machine if you buy it) and try it out. If it has decorative stitches, try them all out. Fill a bobbin and see if it fills evenly or if it's a quick fix if it doesn't. If you plan on quilting with it, take some quilt sandwiches. If you FMQ, do the feed dogs drop. And then, how heavy is it? This is only important if it won't have a permanent home and you have to move it around. Ask when the last time it was serviced, and don't be afraid to open it up to see if it is clean.