select * from sys.sysprocesses where dbid = DB_ID('Test')
(Replace 'Test' with the name of the database you are trying to drop) This will tell you which processes are using it.
If you still want to force drop then, the ultimate approach is:
USE master;
GO
ALTER DATABASE Test
SET SINGLE_USER
WITH ROLLBACK IMMEDIATE;
GO
DROP DATABASE Test;
Hope this helps !