You can determine how much CO2 is in your tank by weighing it but not by looking at the high pressure gauge. A CO2 tank contains liquid CO2 with gas on top. (Much like a Liquid Propane - LP - tank.) As long as there is liquid in the tank, the pressure in the tank is primarily a function of temperature.
The Tare Weight (TW) is stamped on the neck of the tank. That is the weight of the empty tank and its valve, without the regulator attached. My 5 lb CO2 tank has a TW of 7.7 pounds. Full it weights 7.7 + 5 = 12.7 lbs.
Except when the tank is very full or nearly empty, at 70 degrees F the CO2 high pressure gauge will read about 850 psi. At 40 degrees F, it will be around 600 psi. That pressure won't drop until the tank is almost empty and there is very little liquid CO2 left.
So, you can determine if something is leaking by tracking the weight of your tank. You can find a leak location with some very soapy water around each connection - a leaky connection will bubble.
On the low pressure side, the CO2 is all gas, no liquid.