pull the carpet up on the pass front, then have a hose run over the windshield, if its leaking there it will run down the floor/foot board by the hvac fan and box.
my car has a extra layer of floor covering at the furthest away and up towards the ac fan box. it lifts up and will be wet under it if it leaked.
in the pass side floor is a rubber plug, if thats missing the foam under the rug can get wet as well.