What is the most accurate method of testing batteries for A1278 MacBook Pro?
Battery SOC is always time dependent.


  Determining the state of charge (SOC) in lithium-ion batteries is
  essential yet challenging due to the great variability in capacity not
  only across different cells, but also in the same cell. As a Li-ion
  cell ages, it loses its ability to store charge. Consequently, even if
  fully charged, an older cell would deliver usable voltage for a
  shorter period of time than a newer cell.


While in the first moments it might look OK, soon after it shows the true State of Charge.

That is because it measures Voltage and Current variations over time to determine the SOC.

I recommend at least 10 minutes of testing of allowing the Mac to collect the information.

The State of Cycle count is only a indicator for us humans, but not accurate to determine the true state.

Probably the best indicator is the   


  Full Charge Capacity (mAh):   6705 


Found in About this Mac > System Report > Power

It basically says how much charge the battery can hold.

It is not a linear scale thus when it reaches half a way of new battery (4000) Mac will say service the Battery.

New battery has values of >8000. 


  What is the most accurate method of testing batteries for A1278
  MacBook Pro?


it is TIME allowing it to collect the information time related.