Check needle positions, height, relationship to hook and needle plate, left and right, front to back. Check pendulum timing. Check feed dogs, height, alignment, and timing. Check presser foot height, and alignment. Check function of pattern mechanism.
Test sew on machine.
Adjust tensions as needed, check spring, tension release, etc. Check bobbin winding mechanism. Check buttonhole, are forward and reverse stitches balanced. Do final sew off, make sure all parts and accessories are returned with machine. In general sergers are more difficult to work on than sewing machines.