I don't know about you but I have/had many computer devices at one time or another I needed to reload the operating system on. More times than not this would be a Windows OS I have a product key for, usually attached to the device, but I do not know the exact Windows version. Typically I need to know if the Windows version is either a retail or OEM version because the keys are different. In this situation the easiest way to discover the exact version of Windows your product key is valid for is by using the application "Ultimate PID Checker". The application allows you to input your product key into a file and run the app against the file. This is used if you do not know the version of Windows your key is valid for i.e Windows 8, Windows 2013 etc...... and is located under the identify Key tab. If you know the version but do not know if it is an OEM or retail key you can use the From Clipboard tab.
Keep in mind there is always a risk when entering some serial or other number meant to be unique. Even though this is an "offline" key checker there is always the risk this information may be transmitted to another source for the intent of stealing your key.
The file can be downloaded here: