In fact, your car does not use "freon"....
Freon is a common name used for R-12 refrigerant. Your car, like all TT's and most cars made after 1992, uses R-134a refrigerant. If your A/C is blowing cold, your refrigerant level is fine. If you're worried about bad things happening when the refrigerant gets too low.... don't. The system has built-in pressure switches. If the pressure in the system gets too low because of a leak, the compressor will not run, and you won't get any cool air in the passenger compartment. Then you know to get the leak fixed and the system recharged.