How to know which one is used when charging a MacBook Pro with usb-c from multiple power adapters? There is this great question: What happens if you plug more than one charger in the new MacBook Pro (2016)? 

It doesn't tell me how to find out from which source the MacBook is charging though.  Right this moment I have a power bank and a MacBook charger (small, not proper wattage) connected to my MacBook Pro and my battery is at 3%. I'd like to know which one I can unplug.

Is there a way to know which port is giving the power? I'm looking for a precise answer, e.g. from a command line tool. It'd also be good to know with exactly how much Watt it's charging.
I don't want to rely on whatever is indicated on the charger.
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.