How to find out the user of a calendar object

I really would love to build a response to a freebusy-request from outlook.
It seems really easy to find the right calendars for an user, and the format isnt a problem neither.
But how to find the right calendar objects without being logged in as this user????

many thanks for any suggestions!
herzlichen Dank!

Have you tried it with Thunderbird? I am not sure if the calendar app works with outlook.