Use the punctured tube to find where the sharp in the tyre is. It's much easier to find the problem of you are only looking at a small section of tyre rather than the whole thing.
a) when removing the tube, take care not to turn it round with respect to the wheel/tyre
b) pump up the tyre until you can find where the air is coming out. If you can't find it, pump up some more. Tubes will expand up to 6 feet in diameter, by which time even small holes are stretched enough to be findable.
c) mark the hole in the tube - tippex/ballpoint/whatever
d) let the tube down until it's the same size as the tyre
e) line up the valve with the valve hole and closely inspect the tyre where the tube marker is
Feel free to turn the tyre inside out, or to poke down small cuts feeling for any small fragment of glass that may be lurking within the thickness of the tread.
Probably the hardest to find is a short length (1-2mm) of wire strand. It may not be long enough to be apparent on the inside, and there may only be a small pinprick visible outside.
