Inflate the tube so that you can find the source of the leak. You can sometimes find the leak by listening for the hissing and following the sound to the hole. A more reliable way is to fill a sink with a couple of inches of water, and then placing a portion of the inflated tube underneath the water, rotating the tire until you’ve watched the entire tube go through. The leak will give itself away by the bubbles it produces when its section of the tube goes underwater. You can circle the spot or mark it with an X. Otherwise it can be easy to lose.
Even when you think you’ve found the leak, be sure to still check the entire tube, as there may be more than one puncture. Leaks that occur at the base of a valve stem or along the seam of the tube are usually impossible to repair.
If you are out on the road, you can find the leak by dipping your tube in a creek or puddle. If no other water is available, moisten your fingers with saliva and rub lightly over the surface of the tube until the source of the suspected leak is located.
This is an important step. If you cannot find the leak, you will not be able to repair it.