In Exchange 2007 and earlier, after online defragmentation occurred, you could find your database whitespace by examining the application logs and viewing event ID 1221.
This is a pain to do, but luckily for us, Shay Levy posted an excellent article with a PowerShell function for Exchange 2007:
http://blogs.microsoft.co.il/blogs/scriptfanatic/archive/2009/08/13/exchange-white-space.aspx
That works great for 2007, but in 2010 there are no more 1221 events,so Microsoft has provided a much better method of finding your whitespace.
In the Exchange Management Shell, enter the following one liner:
Get-MailboxDatabase -Status | Select-Object Server,Name,AvailableNewMailboxSpace
Unlock-PowerShell 