Trace the water before the repair
Separate the visible damage from the route moisture used to reach it.
A stain tells you where water became visible, not necessarily where it entered. Water can travel along framing, pipes, masonry, and finished surfaces before it reaches the spot you notice.
Start with timing. Note whether the mark appears after rain, after a shower, during snowmelt, or without any obvious event. Then look above and outside the area for roof edges, plumbing fixtures, window joints, grading, and other plausible paths.
Record what changes without opening walls or taking safety risks. A clear timeline, a few photographs, and the conditions that make the stain appear give a qualified professional a much better starting point.
A ceiling mark below a bathroom may come from a supply line, a drain, failed shower sealant, or rain entering farther away. Whether it changes after bathing or after a storm narrows the search.
Apply this nowPhotograph the mark with a ruler beside it, note the weather and recent water use, then check it again at the same time tomorrow.
Do not open electrical areas, climb onto a roof, or treat hidden mold or structural damage without qualified help.