You would have to recover the refrigerant and weigh it.
0Psia
This is most definitely not a DIY task. To do it properly, you need to first test the purity of the refrigerant, which takes special equipment to do, as does actual recovery and storage of the refrigerant.
Use a vacuum pump that pulls it into a tank.
Take it to a shop that does ac work and have them recover, evacuate and recharge the system but the question I have is why do you want to change the refrigerant?
The process that requires you to recover refrigerant from a system and store it in an external container is Recovery. This involves removing the refrigerant from the system and transferring it to a separate container for proper handling and disposal.
The only legal and safe way to do it is to have a shop recover the refrigerant with an RRR (Recover, Recycle, Recharge) machine. Try your local community college if they offer a vocational program for mechanics... they might even pay you for the refrigerant they recover.
You recover the refrigerant with an AC recovery/recycle machine.
The EPA.
Ultimately, the only real way to know for certain is to evacuate the system, recover the refrigerant, and weigh what's recovered. Extremely high pressure on both sides of the system is a good indicator of it, as well. If you don't have the means to recover refrigerant or a manifold gauge set and the know-how to use it, cavitation (shaking) of the AC system when it's running is another telltale sign.
In order to make it easier to recover refrigerant.
Short answer is that you don't. If you need to do this, then you have a certified shop recover and store it.