How do I tell which codec was used to compress a file, and what format a file is in?
There isn't a way to determine with absolute certainty the codec used to compress a file, but the following are your best options:
To determine what codec was used with a specific file, play the file in the Player, if possible. While the file is playing, right-click the file in the library, and then select Properties. On the File tab, look at the Audio codec and Video codec sections.
Use a non-Microsoft codec identification tool. To find one, search for "codec identification tool" on the web. You'll find several tools as well as useful related info.
You might be able to tell the format of a file by looking at the file name extension (such as .wma, .wmv, .mp3, or .avi). However, there are limits to this approach. Many programs create files with custom file extensions. And it's possible for anyone to rename a file without changing the file's format. A file with an .mpg or .dvr-ms extension, for example, is usually just an AVI file that's been compressed by using some version of an MPEG video codec.