If you have more than one storage group (SG), more than one message database, or if your database is greater than 16GB, you have Exchange 2003 Enterprise.
Another way to verify your Exchange Server edition is to open the Exchange Server Setup Progress.log file, which is located on the root of your system drive. Search for one of the following entries:
[16:53:50] Setup configuration information: — ID:xxxxx —
[16:53:50] This is a(n) Standard version of Microsoft Exchange Server
[16:53:50] Setup configuration information: — ID:62227 —
[16:53:50] This is a(n) Enterprise version of Microsoft Exchange Server
— ID:62232 —