You will be guessing how much is in the system without a machine to measure. You can but cans of 134a freon with a hose that will attach to the low side(service port should be on the accumulator) if you want to take a chance. Sometimes there is a small leak and it will work for awhile. If the system is empty you will need more than one can and the a/c system needs to be evacuated to function properly. Moisture is the enemy of freon and the systems function.
the high pressure side and the low side have 2 different sizes so you cannot make a mistake. low side is on the thicker line and close to the evaporator.