Leaks can be detected with special additives that glow when viewed under black light. Any competent a/c repair shop should be able to find a leak. Detector kits used to be sold in most auto parts stores, but I haven't seen any since R-12 started disappearing.
According to a fellow I talked to at Classic Auto Air, hoses that have been used with an R-12 system will eventually develop some sort of internal coating over time that will prevent the R-134a from leaking.
I don't know if I'd go through the trouble of converting without replacing the hoses, but I did hear a similar statement on a TNN show called "Two Guys Garage" (formerly Crank and Chrome, formerly Shadetree Mechanic) when they did an R-12 to 134a conversion. They kept the old hoses and didn't mention any issues with leaks.
The guy at Classic Auto Air did say that 134a works better with a different oil than the mineral oil that is typically used in R-12 systems, though. He said something about it not mixing as well with mineral oil, which often causes bubbles in the sight glass that can make it look like the system is undercharged.
Anyway, I'd give Classic Auto Air a call if you'd like some tech advice: (813) 251-4994. They're in Tampa, Florida and have always been very helpful when I've had questions.
