If you look closely around the neck of your CO2 cylinder you will find a "Tare" value - the weight of the empty cylinder. You can use that with a decent scale to determine how much CO2 by weight remains.
Yes, you definitely bled the cylinder out prematurely - typically a five pound CO2 cylinder can carbonate numerous kegs - like in the 10-20 range - before exhaustion. I would start at the cylinder coupler: there should be either a captured gasket or O-ring at the tip of the regulator's coupler stem, or more commonly there's a bare metal end that requires a separate gasket, which can be either plastic or fiber, either will work.
Assuming there's some type of gasket/o-ring in place, the most likely leak will be at the keg. Still, once you get your cylinder refilled, hook it up with the regulator output shutoff closed, turn on the gas and paint all of the threaded connections into the regulator body with bubble fluid (50/50 water and dishwashing liquid) and make sure there are no leaks at that level.
Then, take note of the high pressure gauge reading, close the cylinder valve and let the system sit for awhile, and see if the high pressure gauge has dropped significantly over time. It shouldn't
If you get through that point the only thing left is the keg - which is always the most likely cause of gas loss. The quickest way to figure out where the keg is leaking is to hook it up and stick the post end in a sink or bucket filled with water. That will expose lid leaks, post leaks and poppet leaks...
Cheers!
