Not all moisture in a basement indicates a major leak or foundation problem. Often, the damp inside the basement actually comes from indoors, whether it's high humidity or condensation issues. This means you want to be sure there really is a leak before you take any steps to waterproof your basement. The following are signs that the problem leading to your damp basement is an external leak.
Sign #1: The corners are damp
Water usually takes the path of least resistance. This means it will permeate the basement walls in areas that are already structurally weaker. In most cases, this will be where to concrete surfaces seam together, such as the corners of the wall or where the wall meets the floor. If you have cinderblock walls, then the mortared joints between each block could also be prone to a leak. If the corners and seams are more damp, then chances are you have a leak.
Sign #2: Wet weather makes it worse
Rain and snow will naturally increase the chances of damp in the basement, just from the rise of humidity this weather causes. Yet this should only be a generalized damp that permeates the entire space. If you start to notice localized water buildup when the weather outside is wet, then flooding might be the cause. For example, soft furnishing may feel slightly moist due to increased humidity, but a saturated section of carpet means water is coming in from outside.
Sign #3: Paint is starting to peel
In a finished basement, it isn't uncommon to paint the concrete walls or to install drywall over the concrete. When moisture begins leaking through the walls, then the paint or drywall suffers first. You may notice that the paint is beginning to bubble or sag. It will then eventually peel off completely. In the case of drywall, it may just gradually soften and then collapse.
Sign #4: You find pools of water
Humidity and condensation can cause a heavy mist-like layer of moisture to build up on the basement walls or floor, but large pools of water aren't usually common. If you find pooling water, you likely have a leak. This is especially true if you see evidence that it flowed, such as a damp trail from a corner to a pool of water in the basement low spot.
Fortunately, you can fix a damp basement. Generalized damp from humidity can be remedied with a whole house humidifier. For exterior leaks, cracks and damage in the foundation have to be repaired. Then you will want to have the basement sealed with either interior or exterior waterproofing. Talk to a basement waterproofing contractor for more help.