Bring your board/ski/foil/whatever & ride behind it. Fill the ballast, drain it, drive it like your going to for the next 10 years. Turn everything on & off. Check all the gauges. Check all the water hoses for leaks. Check the trailer brakes. Hook it to your own truck & pull it.
Tell the dealer you need a fire extinguisher & some cheap orange vests. And some floaty keychains. Most dealers give that kind of stuff away.