If you can't take an exclusive lock on the db, try kicking off users out of this db (make sure by running sp_who2) nobody is using that db, place it in single user mode, and then run consistency checks on that. Thoroughly check the output of dbcc for any errors noticed. Once, the checks have been completed, revert it to it's original state. If possible inform the users before carrying out dbcc checks, becoz of this you won't likely need to take the db into single user mode, as there won't be any live connections to the db.

Regards,

Faisal