How do I know when my CO2 cartridge is running low?

Velocity starts to drop noticeably and the gun gets quieter shot-to-shot.

On semi-autos, the slide may stop locking back or the action may fail to cycle. On bolt-action rifles, you will feel weaker recoil and see noticeable pellet drop on target. When that starts, fire any remaining shots into a safe trap or release the rest of the gas, then change the cartridge. Do not store the gun with a partially used cartridge for extended periods.