Before he can answer, ask when the machine will be back in service. The machine should be as dirty as possible. A mixture of oil and pencil sharpener shavings works well. If the machine has electrical components, add staples and paper clips. A person who has never seen the machine before is preferred.
Bad breath is a plus.
Do this when the machine is in 800 pieces and spread out on the floor. A good service man can fix it blindfolded. If the service man is looking at a schematic diagram, ask if he knows what he is doing. Also mention that you fixed your toaster last week without a schematic. When the repair is completed, tell him what a swell job he did.