If you suspect you have a leak inside your walls, then there are a few ways you should be able to tell right away. The most obvious of these is to see if there are any visible signs of water coming through the walls. If you see water or condensation, then there is likely a leak inside your walls.
Another indication that there is a small leak and/or excess moisture is the development of mold and mildew. Mold and mildew thrive in consistently damp environments.
Similarly, peeling paint or wallpaper in places where it is not normally peeling could be a sign of moisture in the walls.
Lastly, if the wall appears warped or buckled in any way, it is possible that a leak and excess moisture could be the underlying cause.
In any case, at the first sign of moisture, call a professional to have the area inspected. Even small leaks can quickly cause serious damage, resulting in high repair costs if left unresolved.