So, how do I test the functionality of the ADFS?

The ADFS uses the IIS to host his own end points. There is also a simple Login-page that every user can use:
 

https://{ADFS-FQDN}/adfs/ls/IdpInitiatedSignon.aspx


Afterwards a simple „Login-Page“ appears – after one click on „login“ you should see something like 'you are signed in'.