And some h/w stores sell moisture meters. It might give you a trail to follow.
Troubleshooting will be easier if you can coax the problem into appearing, like with a garden hose.
The time lag between the water flowing and the problem appearing may also be a clue.
Here's some more plans of attack
BSD-114: Interior Insulation Retrofits of Load-Bearing Masonry Walls In Cold Climates —
In the link at the bottom, especially look at
BSD-108: Investigating and Diagnosing Moisture Problems