Water leaking onto your ceiling is generally brought to your attention by one of two signs: Either you see the discolouration the leak has made in your ceiling, or you hear water dripping onto the ceiling. This is apt to be big trouble. Even if the water doesn't break through into the room, the build-up of water on your ceiling or in the walls can cause mould. Finding the source of a ceiling leak can be a complicated process, and sometimes cannot be reasonably done. The evidence you see on the ceiling can be far from the actual source.
Visually inspect the floor above the ceiling where you notice the leak. Start with the area directly above the leak, looking for dampness or any active source of water. Move out from that location, eventually inspecting the entire floor. Inspect all water-using appliances and taps to be sure that the entire area around them is dry. Move these appliances out to check behind them, just to be sure.
Venture into your attic while the leak is active. If the water is leaking in from outside, you should be able to find the source in the attic. Move around the entire attic, shining your flashlight over everything, including roof, rafters and beams. The light from the flashlight will reflect off water, helping your search.
Send a helper onto your roof with a garden hose. Have him spray water on the roof, simulating rainfall. Stay in the attic near the area where water is being sprayed, so you can see any water leaking in.
Spray the water on high-risk areas first, such as the flashing between your shingles and a wall, around chimneys and in vents. Once you have exhausted those possibilities, spray the water over the entire roof area, one small section at a time. Start low on the roof, and work your way up.
Remove the ceiling where the leak showed up, and trace the line of leaking water back to the source. This option is not always available, as you may not be able to remove the ceiling without considerable damage to your house. Many of these leaks will be traced to a wall, which will require more destruction as you try to find the source. Use this option only when the ceiling is leaking constantly and you believe the cause is something such as a leaking pipe somewhere inside the house. This is a last resort, taken because this leak urgently requires attention and the problem can't be resolved by other means.