I've had a number of leaks in this house, unfortunately. We face Elliott Bay and the wind driven rain loves to come right in. We rebuilt brick walls, put up gutters, added flashing, re-did the stucco, tuckpointed... it went on and on through many projects (and loads of $$$). I dread the sound of dripping.
I tried a few waterproofing experts but didn't get very far with them. Experienced contractors know how to properly protect a wall, but that doesn't mean they can trace a leak. It can be very difficult to determine the original source.
Ultimately we just rebuilt the parts of the house that were leaking, and we picked contractors who would stand by their work if things leaked again. I wish I had better news for you.
The other thing that works for us is caulking the trouble spots every year. That's a $200 job that might forestall a $10,000 leak.