Just to make sure someone doesn't misunderstand Larry's idea, the stethescope works very well. Remember you take the bell device off the end and put the open tube end up to areas you suspect are leaking. You will be able to clearly discern where the leak is. For those areas that are hard to reach it will also work with an extension on the tube, say a 3 foot piece of copper tubing.
Another trick I used to use with air tubing which was inside a protective sleeve, (roadranger shift knobs, some brake lines etc.) was to take a small piece of paper about 1/4" wide and a couple inches long and put it up between the coupling and the sleeve whichever way the paper blew told me which end of the line had the leak.