Nextcloud CalDAV integration: how to validate connection works?

Configuration
1

Hi there, I have a section in my configuration.yaml which defines access to a Nexcloud instance as follows:

calendar:
  - platform: caldav
    username: <user>
    password: !secret <password>
    url: https://<name.tld>/remote.php/dav/calendars/<user>/ 
    calendars:
      - '<calendar1>'
      - '<calendar2>'

which gives me a sensor, too:
calendar.<calendar1>

when I now go on “info” or “history” of that sensor, it is always shown as green bar, regardless whether i change the password within NC, or try to access a nonexisting <xyzcalendar>.

How can I validate that the external NC URL can be reached, and the user/pwd really connects to it?

Thx for any pointers.