How to find punctures in inner tubes? When I have a flat tire, I find punctures in inner tubes using three techniques, all starting with the inner tube outside the bike:


Hearing where the sound of air escaping originates.
Feeling the escaping air with my hand.
Putting it inside a container of water and seeing the bubbles. I end up using about 10 liters of water, and I have to rotate the inner tube to pass all of it through the water.


Is the method 3 smart? Could water get inside and damage the valve? All three methods seem to work very fast. Is this just a matter of taste or does some miss some punctures?
Do whatever works, really. 

The problem with the container of water approach is that it requires a container of water. It won't damage the tube. Also, if you need to patch the tube, you have to wait for the wet tube to dry. So, I'd generally recommend doing this last (usually leaks are not subtle enough to need the immersion), but no harm going there first. 

The usual technique I use is to try to blow up the tube a bit and put the tube near my upper lip if I can't find the leak by ear or feel it around the tube with my hands; the upper lip area is pretty sensitive, so its pretty easy to detect it. Try to be systematic about it, like start with the valve and go clockwise. If you jump around all willy nilly, you'll take longer to find it. 

Sometimes you get leaks that are too slow for that, and generally in those cases, you end up replacing the tube anyway. 

You can also trace the leak by checking near the valve (a common culprit; if its the valve itself in a Schrader valve, you can spit on it and check for bubbles) as well as running your hand along the tire to see if anything is there. If you make sure to align the label of the tire (e.g. where it says "Continental" or whatever) over the valve, its easy to work back to where the leak is if you find something (since the label and valve are in the same position, the position of what you found relative to the label tells you where the leak is on the tube). 

See this page as well. It also covers what leaks mean stuff (if you have a thorn in the tire, chances are that's your cause, for example. Snakebites are often insufficient tire pressure. If you have a leak on the rim side of the tube, chances are theres something off with your rim tape, etc.).