Determine which SharePoint Version the Site

Basically we can go with step wise as initial step looking for default css files. 

• SharePoint 2013 will have corev15.css. 
• SharePoint 2010 will have corev4.css

If you dont find those , please click F12 and open up the Developer tools. In that go to  Network tab and look for MicrosoftSharePointTeamServices in the response headers of the site.

• For SharePoint 2010 sites --> MicrosoftSharePointTeamServices --> 14.0.0.6114 
• For SharePoint 2013 sites --> MicrosoftSharePointTeamServices --> 15.0.0.4420
• Below is detailed report for MOSS 2007

12.0.0.6535 MOSS 20071 or WSS 3.0 SP2 + December 09 cumulative Update (KB960010 + KB960011)

12.0.0.6524 MOSS 20071 or WSS 3.0 SP2 + 15th December Update (KB977027 + KB977026)

12.0.0.6520 MOSS 20071 or WSS 3.0 SP2 + October 09 cumulative Update (KB974989 + KB974988)

12.0.0.6514 MOSS 20071 or WSS 3.0 SP2 + August 09 cumulative Update (KB973400 + KB973399)

12.0.0.6510 MOSS 20071 or WSS 3.0 SP2 + June 09 cumulative Update (KB971538 + KB971537)

12.0.0.6504 MOSS 20071 or WSS 3.0 SP2 + April 09 cumulative Update (KB968850 + KB968851)

12.0.0.6421 MOSS 20071 or WSS 3.0 SP2 (KB953338 + KB953334) [Updated 1st Aug 09: SP2 download now includes the hotfix (KB971620) that corrects the activation issue more information on the Microsoft SharePoint team blog]

12.0.0.6341 MOSS 20071 or WSS 3.0 February 09 cumulative Update (KB961755 + KB961756)

12.0.0.6335 MOSS 20071 or WSS 3.0 December 08 cumulative Update (KB960010 + KB960011)

12.0.0.6331 MOSS 20071 or WSS 3.0 October 08 cumulative Update (KB957691 + KB957693,KB958567 and KB958569)

12.0.0.6327 MOSS 20071 or WSS 3.0 August 08 cumulative update (KB956056 & KB956057)

12.0.0.6318 MOSS 20071 or WSS 3.0 Infrastructure Update (KB951695 & KB951297)

12.0.0.6303 MOSS 20071 or WSS 3.0 post-SP1 hotfix (KB948945)

12.0.0.6301 MOSS 20071 or WSS 3.0 post-SP1 hotfix (KB941274)

12.0.0.6300 MOSS 20071 or WSS 3.0 post-SP1 hotfix (KB941422)

12.0.0.6219 MOSS 20071 or WSS 3.0 SP1

12.0.0.6039 MOSS 20071 or WSS 3.0 October public update

12.0.0.6036 MOSS 20071 or WSS 3.0 August 24, 07 hotfix package

12.0.0.4518 MOSS 20071 or WSS 3.0 RTM

12.0.0.4407 MOSS 20071 or WSS 3.0 Beta 2 TR

12.0.0.4017 MOSS 20071 or WSS 3.0 Beta 2

12.0.0.3111 Office 12 (PDC image - pre-beta) - This version of Office does not have a support link in the Add/Remove programs dialog box.

Will Keep posting if I find any more info on this.