I would bring different types of fabric, sewing supplies: needle, thread, scissors, needle threader and sewing machine oil. If it's frozen you'll know right away, or if it sluggish or noisy, put a drop of oil in the holes, insert a new needle and sew away. You'll get a 'feel' of the machine - how it sounds, foot pedal or knee bar, if light works, bobbin winder works, stitch quality, etc. You can always clean it later. Pack a lunch and coffee, and if it doesn't work out you had a nice day trip. I have a Singer 301, 2 featherweights, and a Pfaff 1222E. I did the above and have been very happy with my choices. I walked away from a featherweight last week, because I didn't like the feel and sound of the motor.
Mary Ellen FL