How to check SharePoint version (level of SharePoint you are running)?

1) Goto IIS, select the MOSS website
2) Right click and select Properties
3) Select HTTP Headers tab and you should be able to see a custom HTTP Header called "MicrosoftSharePointTeamServices" with a version number by its side.
4) Compare with table below to see what updates for MOSS your farm has installed:

• MOSS 20071 or WSS 3.0 Cumulative update (KB956056 & KB956057) 12.0.0.6327
• MOSS 20071 or WSS 3.0 Infrastructure Update (KB951695 & KB951297) 12.0.0.6318
• MOSS 20071 or WSS 3.0 post-SP1 hotfix (KB948945) 12.0.0.6303
• MOSS 20071 or WSS 3.0 post-SP1 hotfix (KB941274) 12.0.0.6301
• MOSS 20071 or WSS 3.0 post-SP1 hotfix (KB941422) 12.0.0.6300
• MOSS 20071 or WSS 3.0 SP1 12.0.0.6219
• MOSS 20071 or WSS 3.0 October public update 12.0.0.6039
• MOSS 20071 or WSS 3.0 August 24, 2007 hotfix package 12.0.0.6036
• MOSS 20071 or WSS 3.0 RTM 12.0.0.4518
• MOSS 20071 or WSS 3.0 Beta 2 TR: 12.0.0.4407
• MOSS 20071 or WSS 3.0 Beta 2: 12.0.0.4017
• Office 12 (PDC image - pre-beta): 12.0.0.3111 (This version of Office does not have a support link in the Add/Remove programs dialog box).