How to detect installed version of MS-Office?

One way to check for the installed Office version would be to check the InstallRoot registry keys for the Office applications of interest.

For example, if you would like to check whether Word 2007 is installed you should check for the presence of the following Registry key:

HKLM\Software\Microsoft\Office\12.0\Word\InstallRoot::Path

This entry contains the path to the executable.

Replace 12.0 (for Office 2007) with the corresponding version number:

Office 97   -  7.0
Office 98   -  8.0
Office 2000 -  9.0
Office XP   - 10.0
Office 2003 - 11.0
Office 2007 - 12.0
Office 2010 - 14.0 (sic!)
Office 2013 - 15.0
Office 2016 - 16.0
Office 2019 - 16.0 (sic!)

The other applications have similar keys:

HKLM\Software\Microsoft\Office\12.0\Excel\InstallRoot::Path
HKLM\Software\Microsoft\Office\12.0\PowerPoint\InstallRoot::Path

Or you can check the common root path of all applications:

HKLM\Software\Microsoft\Office\12.0\Common\InstallRoot::Path

Another option, without using specific Registry keys would be to query the MSI database using the MSIEnumProducts API as described here.

As an aside, parallel installations of different Office versions are not officially supported by Microsoft. They do somewhat work, but you might get undesired effects and inconsistencies.

Update: Office 2019 and Office 365

As of Office 2019, MSI-based setup are no longer available, Click-To-Run is the only way to deploy Office now. Together with this change towards the regularly updated Office 365, also the major/minor version numbers of Office are no longer updated (at least for the time being). That means that – even for Office 2019 – the value used in Registry keys and the value returned by Application.Version (e.g. in Word) still is 16.0.

For the time being, there is no documented way to distinguish the Office 2016 from Office 2019. A clue might be the file version of the winword.exe; however, this version is also incremented for patched Office 2016 versions (see the comment by @antonio below).

If you need to distinguish somehow between Office versions, e.g. to make sure that a certain feature is present or that a minimum version of Office is installed, probably the best way it to look at the file version of one of the main Office applications:

// Using the file path to winword.exe
// Retrieve the path e.g. from the InstallRoot Registry key
var fileVersionInfo = FileVersionInfo.GetVersionInfo(@"C:\Program Files (x86)\Microsoft Office\root\Office16\WINWORD.EXE");
var version = new Version(fileVersionInfo.FileVersion);

// On a running instance using the `Process` class
var process = Process.GetProcessesByName("winword").First();
string fileVersionInfo = process.MainModule.FileVersionInfo.FileVersion;
var version = Version(fileVersionInfo);

The file version of Office 2019 is 16.0.10730.20102, so if you see anything greater than that you are dealing with Office 2019 or a current Office 365 version.