Water leaves clues. Start with them.
Note when it happens
During rain, after snowmelt or all the time? Describe what you observe without performing a test that might worsen the issue.
Locate the marks
Wall, wall-floor joint, window or crack: location helps prepare the examination. Ambient humidity does not necessarily have the same cause as a leak.
Have it assessed
A bowing wall, a changing crack or a structural safety concern requires an appropriate assessment before repairs are proposed.
For background on moisture: Government of Québec.