There is only one way to know for sure if you are low on refrigerant (but technically, what you use is called Suva rather than Freon), and that is to reclaim and weigh what is in there now.

Has the system been serviced in the past? It will naturally leak a bit, so if it is the original charge it is likely to be low. Low refrigerant level causes insufficient oil return to the compressor before it causes the system to start to blow warm. Insufficient oil return causes compressor damage.