To be honest, unless something very obvious, somebody will have to get in boot whilst another hoses the car over.
Although it may he hard to believe, but from work related experiences, a hose on a car to try and find a leak doesn't always replicate actual rain water falling from the sky!
Issues with cabriolets for example that were often prone to water ingress sometimes could only get replicated parked in the rain instead of using a hose on a tripod!!