Normally I append .bak or .old to files. The result should look like
In Windows, open any Explorer window (i.e. Open a folder, such as My Documents). On Vista, hit the alt key. Click Tools and then Folder Options. Click the View tab. Just below Hidden files and folders, uncheck Hide extensions for known file types. Click OK.
This will allow you to view file extensions for all files and not just ones without program associations.