Start by looking at the front edge of the door. If it's sticking out beyond the fender, it can catch rain water and overload the door seal.
The next thing is to have a helper run water from a garden hose onto the windshield while you watch inside. If you have a sunroof, check for blockage in one of the drain tubes or if one is disconnected.
Spray water around the door, then open it and check for wet areas on the seal. The Chrysler dealer, (and probably other dealers), has a spray can of white powder for finding leaks. You spray it on one surface, close the door slowly, then open it and check to see if there's a place the powder didn't transfer.