You're never going to get it all, but since the CO2 is being directed downward, and is heavier than air, you'll get most of it. As far as actually checking, the only thing that comes to mind is the match test, but I'd think the force of the gas coming out of the PRV would blow it out regardless.
I just open the PRV, let the gas run for ~5 seconds, re-pressurize, open for ~5 s, then call it good.