How can I tell if my basement walls are leaking?

How do know whether your basement walls are wet due to water seepage or condensation in the air? Try taping a 1 ft. square piece of aluminum foil to the wall with some duct tape. After a couple days inspect the foil. If the foil side facing the wall is moist, you have water seepage into the basement. Find a masonry sealer like ThoroSeal to waterproof the wall. If the foil side facing the room is moist, you have condensation and need better air circulation and or a dehumidifier.