I think the car wash is a good first start on trying to find if there are any leaks. I agree with Terry and Gerd in doing more than just the "car wash" test. This is a good test of watertightness for the upper portions of the car - around windows, vents, etc. I am not sure of the best way to test the underside of the car except to drive it on wet roads in the rain and see what happens. The SAS cars are not VW based so the undersides are configured a little differently.
I do know that driving the car on a trailer in continuous rain from Knoxville to Carlisle did not expose any leakage. I have driven it many times in very bad rain without any leakage.