Using SharePoint Central Administration Web site SharePoint HTML Site Settings admin pages or IIS Manager, on the web sites properties HTTP Headers tab, virtual servers once extended will show the following version numbers: 
    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).

¹To confirm that a particular service pack is install on SharePoint Server you must either check the version numbers of specific dlls as specified in the related Microsoft Knowledge Based article or select the Show Updates check box in Add and Remove Programs.

    WSS 2.0 SP3                                    6.0.2.8165 
    WSS 2.0 SP2 KB900929 + KB924881 6.0.2.8117 
    WSS 2.0 SP2 rollup KB900929           6.0.2.8000
    WSS 2.0 SP2                                    6.0.2.6568
    WSS 2.0 SP2 Beta = R2:                   6.0.2.6551 
    WSS 2.0 SP1 + KB887981                6.0.2.6411
    WSS 2.0 SP1:                                  6.0.2.6361 
    WSS 2.0 Unservice packed:               6.0.2.5530

To confirm that service packs are installed, especially with SharePoint Server 2007, goto Control Panel -> Add and Remove Programs
Select the product and then click: Click here for support. The versions will
be displayed as follows:

See David Hunter's blog post, "SharePoint 2007 Versions ", to find the version number directly from a SharePoint database.

Thanks to Matthew McDermott and Kenneth Searles for the update on the version related to the hotfix KB887981

Note: If you have a version number starting with 10, then it is either SharePoint Team Services (SPTS) or SharePoint Portal 2001 (SPS 2001).

• SPTS, 10.0.2627.01 is the version with no service packs. In the Administrative Tools menu, you will see a link to Microsoft SharePoint Administrator. This will display a web page, where you can see the version number of the virtual server(s), e.g., 5.0.2.2623. The ISAPI filter is fpexedll.dll.
• For SPS 2001, 10.145.4629 is the version number with no service packs. There will be no ISAPI filter, and from the Administrative Tools menu you will see a link to SharePoint Portal Server Administrator which launches a mmc snap-in.