If you have defaults set, database maintenance will run every morning between 1am and 5am. After this maintenance runs you will see an event in the application log. This will be a 1221 event and will detail how much space is in this database.
Event: 1221
Source: MSExchangeIS Public
Type: Information
Category: General
Description:
The database "[StorageGroup]\[MailboxStore] ([ServerName])" has
[xxx] megabytes of free space after online defragmentation has terminated.
You could also dismount the store in question and run ESEUTIL /MS Database.edb.
If you aren't getting this event in your event log on a consistant basis (at the very least once a week) it might indicate that either maintenance isn't running long enough or at all. You should check the properties of the store and adjust the maintenance window as needed.
- Joel