How can I tell if my c02 Revolver (Crosman SNR357) is leaking?

I can only think of a couple of ways. One would be to immerse the whole gun it a pail of water and watch for bubbles a few minutes later. Then after degassing you would need to tear it down and dry it out. Another way would be to count your shots on a full cart then when empty install a new cart let it sit a couple days and shoot it for shot count again. Lower shot count means leak.
 
Thats one thing I always find to be tricky, Like what if the shot count is same but weaker at the end which you cannot tell. When it was brand new i kept shooting until the pellet got stuck in the barrel that was one sign it gave me where it was very weak , after i release the c02 I still Here a short TSsssssssss is that normal? How do you know when your c02 is finished what is the sign you look for without getting anything stuck in barrel. Im Guessing that getting a pellet stuck is not the way to know your c02 is finished.
 
I have a qb78 co2 rifle. When its getting low on co2 the sound of the shot has a distinctly different sound while also being substantially quieter . After the first few shooting sessions with the gun I was able to pick up on this. When it starts to sound different I also notice the shots falling a couple of inches lower than the expected poi at 10 yards. At that point I just dry fire the last bit of co2 out the gun. Hearing co2 still in the tank is normal, co2 doesn't operate at very high pressure so it needs a decent amount to propel a lead pellet through the barrel.