1. You should do a vacuum check This means that with the dust cap securely in place and/or the tank attached to a tank with the valve off you shoudl be able to inhale as hard as you can (about 20 inches of water pressure) and get no airflow at all. Any air flow indicates a potential leak in the first or second stage. Check both second stages.
2. You can then pressurize the reg and they should function normally with no hissing from a slight free flow. Any even slight hiss of escaping air means the seat is probably improperly adjusted if the hiss starts immediately. If the hiss starts after a few seconds to a minute or two, the fault is a leaking high pressure seat.
3. Depress the purge button. If the lever is properly adjusted, only a small amount of depression is needed to initiate air flow and full depression should give you a lot of flow. If you have to depress the button too far to get any flow, it means the lever is too loose and the working range of the valve will be limited, limiting airflow.
4. Cycle the purge button a few times on each second stage. The reg should still be hiss free when you are finished.
5. Inhale from each second stage and the second stage should deliver air when you inhale gently. They should feel about as easy breathing as before you sent them in for service, if not slightly easier breathing.
(1-5 can and should be done as a pre-dive check the first dive of the day. If you will be cold water diving, do the check at home so the reg has time to rewarm before you start the dive.)
6. Shut the tank valve off but leave the tank connected. If you have an IP guage (pretty easy to make with a 14 NPT to quick disconnect fitting, a 1/4 NPT nipple and a Wal-mart variety air tank/compressor pressure gauge) it should show no drop in IP over several minutes. If you don't have an IP gauge, it should still be pressurized several hours later.
6A. Alternatively you can leave the tank valve turned and then submerge the whole regulator, tank and all, into the bath tub (Pony bottles work great for this) to do a leak check. There should be no bubbles escaping anywhere.
7. It's obvious but worth mentioning - with the regulator depressurized, check each of the hoses where they attach to the first or second stage to be sure they are secure. An interuption at the wrong time can cause the tech to forget to tighten the fitting on a just installed hose or second stage.
The above checks will catch 99% of everything that could have went wrong during a service.
