While brewing beer or carbonated beverage, how to check the available CO2 in the cylinder tank?

After brewing for a period of time, one question is becoming urgent for you to learn.
It is how much CO2 still left in your CO2 tank? How could we learn it? Only upon guessing?

After all, it will be better rather than waiting till you completely run out.
 
The best way to resolve it is to adopt a double gauge regulator.
It will tell you clearly how many kegs can be dispensed per CO2 tank, as well as when you need to refill your CO2 tank.