We take moisture readings across the foundation walls and the slab, with extra readings along the joint where they meet, under window wells, around the sump pit and beside the water heater and laundry.
The pattern points to the source. A band of moisture along the base of an outside wall suggests water coming through from the soil. Damp spots only around pipes or the water heater point to plumbing. Walls that read dry in a room with high humidity mean the problem is the air, which needs a different fix from a wall that lets water through.
Tell us what you know about timing: whether the wet line shows after rain, after snowmelt or all year, and whether it began after work on the house. A damp patch that tracks the weather usually means ground water, and one that doesn't usually means a pipe.