In my experience the amount of pressure indicated on the gauge for the CO2 tank stayed at approx 500 PSI until the tank was virtually empty then dropped to 0 in a matter of seconds. So I got no indication of how much CO2 was left in the tank.
Many other posts I've read by others on the forum said they experienced the same.
Here's just one recent post.
https://www.homebrewtalk.com/showthread.php?t=46741
A more accurate gauge is to weight the tank.
The best way to test for leaks is with a solution of star-san or soapy water. Brush it around all connectors, posts, lid, ect and look for bubbles that grow and burst. It's certainly a lot cheaper that watching for your pressure to drop.