First, take looking of the thickness of the power bank, if the thickness of more than 18mm, it is made of 18,650 battery. 18,650 battery specifications have 2200mAh, 2400mAh, 2600mAh, 2800mAh. If the power bank you bought was made of 18650, the capacity must is these multiples of such batteries. Otherwise, if your power bank is made of polymer battery, Li-Polymer battery capacity we can’t know from the power bank appearance, you can buy branded power banks, usually, brand power banks capacity are real.
Deficiency of power bank capacity is very difficult to judge, we generally adopt A grade battery, good power bank must has high conversion rate.