If I'm looking at a "serviced" machine, I'm looking for straight stitches with no wobble, and I'm going to flip the fabric to check the backside to know that the tension is correct.
I want to hear the motor, but not much of it!
I want to see the wiring, and I want to see the bobbin area to know that there aren't nicks and scrapes. I want to see the needle plate for the same reason.