if you ALWAYS take your car to the dealership for services, oil changes, etc... the service writer should know you pretty well, and might give u a freebie on that... but for the most part, if u really want to find out where it's coming in from, you'll have to remove most of the trim pieces from around the wet area... i.e. lower dash, kick panel, door sill, etc... sit inside with all the doors closed, and have someone hose the car down while looking for any signs of where the water came in. I had to do that on a corolla...

also, check to see if the rear floor mats had any signs of wetness... then check the trunk. the corolla came in complaining of the front mats being wet... no leaks in front... then we found the rear mats being somewhat moist... long story short, it was the trunk weatherstrip that allowed water to leak through.