When you log into the NemKonto self-service, you can see which one of your bank accounts has been selected as your NemKonto under "Your NemKonto".
If your NemKonto is in a foreign financial institution, you click on "Foreign NemKonto" on the self-service platform in order to see which account is your NemKonto.
You need to use MitID in order to use the NemKonto self-service.
If you have not yet activated your MitID, you can read more about it at www.mitid.dk.