If you really want peace of mind, the only way is to get the compressor off the car (after having obviously legally recovered any refrigerant inside the system), drain any oil from it, flush the system of any oil left inside it and then place the entire system oil charge inside the compressor (through suction port if no drain hole is on the compressor's body), fit the compressor back on the vehicle and rotate the compressor's shaft by hand (rotate the clutch hub if it has a clutch, rotate pulley if no clutch) 10 times before charging the refrigerant and 10 times after the refrigerant has been charged. Obviously, the engine off type of refrigerant charging process, done through a dedicated recovery station and the high side with liquid refrigerant into a vacuum.
Anything else, and we're talking assumptions (for example, assumption that whoever put the compressor on the car cared about balancing the oil charge or flushing the system and doing what i listed above if necessary, such as with a seized compressor), not certainties.
The only time you want to inject oil through the lines is when you've evacuated the system and a certain quantity of oil came out in solution with the refrigerant (this quantity has to be replenished with the same quantity, chemistry and viscosity of refrigerant lubricant). Anything else, and it has to be poured inside a removed compressor.