The .NET Version Detector does exactly what its name suggests, quickly identifying which versions of the .NET Framework are installed on a PC.
The program is portable, which makes it very convenient to use. Unzip its single executable onto a USB key and you can run it anywhere.
And after you've launched the program, there's nothing to do, really - just look at the panels on the left of the screen, and any versions displayed in white text are installed, while those greyed-out are not.
Or, if you'd like something a little clearer perhaps, scroll down the text box at the bottom of the program window for a simple list of installed versions.
There is one small added convenience, though, in the form of buttons to the left of each .NET version, which are supposed to represent download links. Green buttons are the Framework download, purple the SDK. (If you don't know what "SDK" means, you want the green.)
Version 14.4 adds support for detecting .NET 4.6.