hydro testing

Pa requires one every 5 years to 100% operating pressure, but a leak down check is sometimes done as well (This usually isn't much fun, the inspector is really supposed to ignore a certain amount of seepage past valves as non-critical, once he has located that as the leak, but some fellows won't, and having to regrind the seats on all your valves just to get rid of tiny seeps is a royal pain, some people will get mad at this point and just plug off everything for the test). Propery done a hydro will help to confirm what you have already determined from the visual inspection, improperly done it can even cause actual damage to the boiler.
As for a hydro test being "proof" that the boiler is safe, I know of a couple stories that will illustrate otherwise. First is about a group of fellows who welded a 1/4" longitudinal stay in an aluminum beer can and procceded to do a 150# hydro on it....It held.
The other was about a couple of old boys (not in Pa) some 20 or so years back, who had a state inspector come do an inspection and hydro on a portable boiler. They asked the man if he was satisfied it was good to go. He said yes. They then proceeded to knock a 1/4" hole in the barrel with single blow from a chipping hammer, and said "WRONG!"
There is still no real substitute for a pair of good trained eyes.
IMHO Be it visual, hydro, hammer, ultrasound, magnaflux, or whatever, any safety test is really only as good as the guy doing it.