If I understand AnnieO's response to this same question I posed to her the leaking side panels was observed by her glass installer/repairman. (Don't take that as the gospel truth, my memory is always subject to doubt.
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In her case the leak would present itself by drops of water on top of the left side driver's floor mat and show signs of accumulated dust under the dash having been wet enough to drip, a few drops maybe laying on the floor mat. That alone tends to indicate the leak isn't from the upper A-Pillars---those leaks can and do run down inside the trim panels which cannot be seen. Once the water is below the actual dashboard it runs under the factory floor mat along inner pieces of sheet metal.
In order to observe the upper corner leaks you'll have to run a hose for quite some time while craning your neck to see if water is running. Another option is using some sort of inspection mirror which shows the area where the W/S is bonded to the body. Another "better" option is to simply remove the A-Pillar trim completely. Any year past 2002 does have the grab handles which obviously need to be removed first.
Your planned process of starting low the moving upwards is exactly correct--its how I do it too. A steady stream from the hose without a nozzle or other squiring device is preferred---rain typically isn't driven as much as it lands on something then flows toward gravity. If you don't see a leak immediately keep the water running in for upwards of 10-15 minutes at the bottom.
Once you're ready to move towards the top its okay to spray all across the top looking for leaks there---at this point we're not too concerned with the A-Pillar exterior. If no leaks across the top that's good news.
Testing the A-Pillar is somewhat easy----position the running water stream so it flows down the pillar from the top, water stream positioned just inboard slightly to test 2-3" as well as down the full length of the W/S. This too could take upwards of 10-15 minutes before a leak can be seen.
At this point re-check under the dash again---by now if a leak is present in the area below the W/S cowl area it should show up now.
Lemme know if I can help further.