If it's the little chrome ball valves with the red handles, they do have check valves.
But, if you want to see for yourself, you can take a valve out of the manifold. You can see the check ball at the end of the NPT threaded end of the valve.
Originally Posted by Ecnerwal
What does the primary pressure gauge on the tank tell us? That's right, the temperature. Put it on a scale if you want to know how much is in it...
Put some duct tape over the gauge - Or better yet - Replace the high pressure gauge with a plug - High pressure gauges are useless!