make sure that they charge, sound is right, they are clean, charges with lightning (magsafe if supported too), siri, music control, battery life, the instant device switching feature, spatial audio (if supported), noise cancelling (airpod pros), etc.

also test to see how long it takes for the taps to work. when you tap on it for the controlling features like pause music and answer calls, make sure that it doesn't take a while.