The best place to detect where a leak might be coming from is often the attic since it connects directly to your roof. Make sure you take a flashlight with you since water reflects light. Look around the attic for anything that might be water and then use a colored marker or something else that will leave a visible sign and mark it so you can find later on.
From the exterior, you will need to check around the area of the roof for anything that might have holes or be broken. Check on both sides of the rafters since water might run down them over to the wall. Carefully check around all of the vent pipes that go around the chimney and through the roof.
After it has stopped raining, use a pair of binoculars to check your roof from the ground. You can consider going up on the roof if you feel comfortable doing that. However, if you do, make sure to wear the right fall restraint equipment. Check for water pooling or missing shingles. Quite often leaks will develop around the flashing or at the point where two roofs join together, especially if a second one was added later. If there are leaves clogging up the valley, water may back up past the valley flashing and then penetrate into the roof. That can also occur if the valley isn’t wide enough for all of the water that is running down through it. Sometimes you can narrow down your search by taking your water hose and running water over a suspect and watch to seek if a leak starts up again.
If a leak occurs in a cold climate during the winter, ice dams might form along the eaves of the roof and melting snow may be forced back up underneath the shingles and then down into the house.
If there is water puddling around a specific window only when rain is being blown by the wind against it, then that is the likely spot. That will be a lot easier to repair than your whole roof.