Simple way: get a flashlight and look into the pump exit nozzle. You'll mostly just see the stationary vanes, but you can also look past them and see the impeller a bit. But you'll get a better view by going under the craft (on the trailer) and look backwards through the intake grate. You can see the leading edges of the impeller from there (that's the edge that gets damaged most, and causes cavitation).
Or the hard way: remove and dissassemble the pump housing.