Sometimes, if you’re lucky, you can just see it or hear the air hissing out of it. More difficult punctures can be found by slowly running your hand just over the tube and feeling for an air leak.
Even tougher punctures can be found by moving the tube around right next to your eye. This sounds odd but your eye is very sensitive to blowing air.
The last and failsafe method is to dip the tube under water and watch for the source of rising bubbles. This can be done in a small sink by rotating sections of the tube under water water while the rest of the tube remains in air.