Under normal circumstances, the ET is nearly empty. If you knock on it with something, if it makes a thud sound, it's likely shot.
The ET should be pre-charged to your normal water pressure. Then, it will have the maximum space to absorb any of the expansion and will stretch less, making it last longer.
The tank can be installed in pretty much any orientation, but because when it does eventually fail (they all do), it needs to be properly supported. While the air chamber doesn't usually leak much (the valve can if the cap is poor or not tightened well, or the gasket is shot), you do want access to check it.
A telltale indication of a failed ET is that it is full or water, or, if you try to open the air valve, water comes out.
As was said, pretty much any time that the utility does some work on a meter, they will be adding a check valve, if one didn't exist (and maybe replacing it if it was old or leaking). That immediately makes the system closed, and pretty much requires an ET to avoid issues. Water doesn't compress, so any expansion will increase pressure. That pressure isn't great for much of anything in a plumbing system from hoses, valves, water heaters, etc. A PRV with a bypass valve in today's era of check valves on the supply are a waste of money. Even if they open, there's no place for that expanding water to go that way. It's one more thing that can fail and costs more to buy.
