If water is staining the ceiling or dripping down, the leak is probably directly above. Occasionally, water may travel along a joist and then stain or drip at a point some distance from the leak. Any wall stain is likely to be below the actual location of the leak and you'll probable need to remove part of the wall to find it. Without the sound of running water and without drips or stains as evidence, leaks are more difficult to find. Using a flashlight, check all the pipes in the basement or in the crawl space.
Most outside leaks occur at connections.
One is at the meter and the other is near the point where the line enters the home or building. Sometimes the line leaks between these connections. Look for a wet or soft spot in you landscaping or near the driveway. You can save a lot on the cost of a plumbing job if you do it yourself. In other situations, the choice may not be as clear.