Doing a vaccuum check as outlined above by trying to breath off your reg with the dust cap in place is a very good check to make prior to every dive.
You should not be able to get any air through the second stage at all. A very very small amount of air is probably not worth delaying a recreatiopnal dive for but warrants checking out later - and it may just be due to a less than perfectly sealing dust cap. A noticeable amount of air is sufficient reason to postpone the dive until you find the source of the leak.
Cracked mouthpieces are a common source of leaks, and due to their location in what amounts to a venturi tube, water is drawn in amounts disproportionately greater than what you would expect from the size of the crack.
A pinhole in a diaphragm will cause areg to breath wet and a tear will cause it to breathe extremely wet. Tears are easy to spot once you remove the purge cover, but pin holes will require you to stretch the diaphragm slightly in a radial manner as you move around the diaphragm while holding it between you and a light source. A hole will be indicated by the pin hole sized light shining through the diaphragm.
The downside to this test is that it requires diaphragm removal and improper seating of the diaphragm is another potential source of a leak.
Plastic second stages have a bad habit of cracking with age, use, impact damage, or improper excessive tightening of the inlet fitting during an annual service. A crack will often initially make itself known by wet breathing but they almost always grow and a monor leak at the beginning of a dive can literally be a full blown flood by the end of the dive. Since these leaks are large, and often sudden they can result in the diver aspirating a significant amount of water with a resulting gag/cough/inhale cycle starting that can make it very hard to even switch to the octo. Careful visual inspection of the case will usually be adequate to find a case crack. Look most carefully around the inlet fitting and around any other area of the case with holes or potential stress risers in it. Often, you will see dried salt in or around the crack before you will see the crack. Case cracks along with torn diaphragms, are the type of leaks that can potentially be dangerous underwater and are the ones that would prompt me to cancel a dive.
There are anywhere from 1 to 4 o-rings sealing various openings in the second stage case and they are also a potential source of leaks. In most cases however you'll probably want a tech to change them for you. On regs so equipped, a loose flow vane adjustment knob, usually means a hardened or worn o-ring that is probably leaking. Otherwise diagnosing them as a problem, outside of an annual service, is usually a case of checking everything else and still not finding the leak.
