There are a number of signs you can watch and listen for that indicate you may have a refrigerant leak. Keep in mind, some of these signs can indicate a different type of problem with your AC, but the point is you should give us a call if you notice any of these!
Poor Cooling - When there isn't enough refrigerant in your air conditioner, it's going to struggle to actually keep you cool.
Higher Utility Bills - When your energy bills seem to spike in correlation to your air conditioner use, even though they weren't this high last summer, it usually means something is causing the AC system to work inefficiently. It could very well be due to a refrigerant leak.
Humid Indoor Air - When an air conditioner is working as it should, it has some dehumidification properties. Note, air conditioners aren't designed to be dehumidifiers too (unless they have a humidistat installed with them), but if you notice your indoor air seems more humid than usual when you run your AC, it may be because you have a refrigerant leak.
Longer Cooling Cycles - When refrigerant is leaking, your air conditioner will run longer to try to reach the temperature you set on the thermostat, and it will struggle to do so.
Ice on Evaporator Coil - Ironically, an evaporator coil cannot absorb heat if there isn't enough cold refrigerant going through the component. Therefore, ice can form on the evaporator coil. This has to be removed and the problem addressed by a professional.
Bubbling or Hissing Noises - Depending on if it's in liquid mode or gaseous mode at the time, leaking refrigerant may make a bubbling or hissing sound.