Sometimes while troubleshooting Windows Media Player, you want to check which codecs are installed on your PC, you have the right codecs or not before installing the codec you need. For that how to find which codecs are already installed?
Open Windows Media Player, From the Help menu click About Windows Media Player. If Help Menu not visible, right click on top bar (below the title bar) click on Show Menu Bar. Or just press Ctrl+M.
Click on Technical Support Information and it provides detailed information which includes OS info, lists Media Player and other Binaries, lists Audio codecs, Video codecs , MPEG/DVD Filters, plug-ins, services etc.
So this way you can know the installed codecs. The screenshots are of Windows Media Player 12.0.9431.0 in Windows 8.1 Preview with Media Center.