How to know which one is used when charging a MacBook Pro with usb-c from multiple power adapters?
The one that has the most power is the one that does the charging

From Apple's support page Connect with Thunderbolt 3 on your new MacBook Pro (that was linked in the question), it states that


  
  If you connect multiple power supplies to your MacBook Pro, the one that provides the most power will be used, regardless of the order in
  which you connected them.
  


You need to check the specs of the power bank and the charger.  How many watts to each have.  To get this info, use the simple calculation:

Volts = Watts x Amps


If you power bank give you 5V at 2A it's capable of delivering 10W.  If your charger is capable of 12V at 5A then it's capable of delivering 60W.  The one with the most is charging your Mac.

Most likely, your power bank is nowhere close to an AC adapter - that's the one you should disconnect.