its basicly just like test driving a car. your looking to see if the motor run correctly and there are no problems on the water with the ski/boat. because they run diffrently in the water than on the trailer
What he said. Take it out, let it get up to full operating temp. Try every button and function. If it has a major mechanical issue, you should find it. I would go out at least 30 minutes. Any serious seller should understand.