How do I stress test a roof for leaks?
If it is a shingle roof, a pressure washer is going to tear it up. Barrel tile is another victum of presure washers unless it is a whole new roof. Old tiles will disintergrate because the bonding agents used to make them grow less adhesive as time goes by.

You should only use you garden hose and let the water run from at least three courses above the old leak, higher would be best though.

What happens most of the time is a simple fix when the roof is first put into place. (Make sure they lap the felt correctly, and do not leave any holes without tar being applied liberally in the right spots, meaning the hole and 4 inches beyond it, then the felt patch and some more tile. They should also slip the top of the patch under the top of the tear in the felt.

Also make sure they are using Simplex nails, not regular thin tabbed nails. Those will suck up much of the time causing a leak to just appear out of no where.

One thing to keep in mind, "Water does not Run Up Hill". in 35 years of construction I have heard that many times on the job and it is crock of Whale snot.