How to detect DBCC ShrinkDatabase completion percentage?

Well, you shouldn't ever be using DBCC SHRINKDATABASE, IMHO - if you need to shrink files at all, you should think twice, maybe even three times, and even in the odd case where it really is warranted (hint: this should be rare), you should target each file individually using DBCC SHRINKFILE. Please read every single word on this page.

Anyway, you can see the progress using:

SELECT percent_complete, estimated_completion_time
  FROM sys.dm_exec_requests
  WHERE session_id = <spid running the shrink>;

Documentation for sys.dm_exec_requests.

No need to reinvent the wheel, Adam Machanic has written a usefull stored procedure to replace SP_Who and SP_Who2. You can find it here for free.

(http://whoisactive.com/)

This stored procedure has a column “Percent_Complete” which gives you how much % shrinking of database is been completed by Server.