It's been the Nth time I've had to checkup the version of SharePoint that is installed on a Farm - and I am now finally keeping this here for my own notes.
All these times I've been hitting back on a very nice post by Penny Coventry that contains a list of SharePoint Patches and respective version numbers:
The easiest ways I use to check version numbers are:
1. In Central Admin, Site Actions > Site settings:
2. In IIS, right click on the web application and select properties > 'HTTP Headers' tab, under Custom HTTP Headers section: