Are you after the actual version?
The easiest way is to check what version of MS Project Professional you run.
If it is 2003 that you connect with, than it is Project Server 2003, if it is
MS Project 2007 it is 2007.
Or which SP you run?
If so you may want to go on the app server go to Add or Remove Programs, or
check the database version whith the following number:
Service pack 2 has the number 12.0.0.6422.1000
April Cumulate update have number 12.0.6503.5000
Hope this helps