By listening with your ears! Measurements (most of them) are worthless. I need to know how efficient the speaker is so I know if I’m going to need a different amp or if my existing setup will work. If I like a pair of speakers that are 65db efficient, I will need a larger more powerful amp.
As for most measurements in audio, when have you seen measurements for the new car you are buying? You might want to know mpg and if it will tow a trailer. The manufacturer might state the car has 300hp, but does the new owner really know if the engine measures 250 hp or 310hp? We don’t, and we don’t care. On the test drive, if the car responds to my right and left feet and my 2 arms on the steering wheel, I’m good to go. I don’t need to know if the hp is lower than specified, or cylinder leakage is out of range or what the vacuum pressure is?
why would we care if distortion of 1 amp is .006% and the other amp has . 00.005% distortion. Like we can hear that.