halocline
Contributor
Regs function without free flow, no cracked hoses, diaphragm good working order etc. Same with BC, tanks, etc. Well maintained, hold air, no leaks, etc.
Basically, it will function through the whole dive without mishap or malfunction.
It's a bit of a crapshoot, but remember that with the more popular dive ops the rental gear gets used constantly, so typically if there's a problem it gets fixed. Regs that work on the surface will usually work fine for a dive, so probably the best thing you could do would be to connect the reg to a tank, try a vacuum test before pressurizing, then pressurize and check for leaks in the rinse tank. Just a general inspection, if you got really anal you could bring a small IP gauge and check that for stability. For the BC you could just inflate a bit and toss it in the rinse tank.
The usual suspects for small leaks are tank o-rings, SPG spools, and LP inflators, so you could pay some attention to those spots.
Come to think of it, it's more-or-less the same thing a responsible diver would do with his/her own equipment, pre dive.