I guess you have to assume the diver has also done an equipment check, like are the valves open. Does he have his fins. Mask. Is his suit zipped. There is no ONE check you can make....including breathing off the reg.The SPG not wavering when a reg is breathed shows the valve is open, it does not mean the valve is fully open. Depending on the valve, 1/4 turn open may be enough to pass that test.