It should be in the condenser name plate. It should said, "Factory charge 5 lbs."
How does technicians calculate how much refrigerant does a system need?
We use factory charts or by Subcooling and superheating. Or we use the calculation slide card.
The calculation slide card had two sides. The front side has the Title "Required Superheat calculator (cooling, Non - TXV)" and the back side has subcooling calculator (cooling, TXV). This is for R-22 only. You'll need different slide card for R 410A.
There are tools you need and information. You might as well buy air conditioner books. It probably takes me three or four pages tell you.