for the very slowest punctures hold the tube submerged (for which you will need weights/mesh) in soapy water. By the time you have made/drunk a cup of tea, the puncture will usually have made itself known by leaving a tell-tale trail of bubbles behind.
There is (of course) a type of puncture which can defy any attempts made to locate it at low pressures. The reason for this is very simple; at low pressures there is no puncture, no leakage of air to be seen. How does this happen? Well, usually a patch lifts, lets the tyre down, then reseals itself. When the puncture has nearly driven you barmy, or at first, you can resort to the 'puddle test'. In this, the fully inflated tyre is partially submerged (eg. in a puddle), and because the pressure is so high, bubbles will come out. Unfortunately the tyre's outer cover can be very nearly airtight too, meaning that where the bubbles come out is only an approximate indication of the puncture's true location, but at least this simple test may stave off puncture-induced barminess.