GSpot is a nice little tool that allows you to see what audio and video codecs the video file uses in order to determine what you need to install on your system to watch it. Program supports OGM, MPEG, RealMedia, Windows Media, Flash and AVI

Gspot Codec Information Appliance Features
Establishes what video codecs (audio and video) are required to play an AVI file.
Determines whether these codecs are installed on your system.
Isolates problems associated with these codecs.
Simple operation - Basic use: "File > Open", then read results.
Identifies download induced problems (truncated files, "cooked" files, etc.)
Shows framerate, duration, aspect ratio, bitrates, AVI structure info, and more.
Displays and allows editing of RIFF info (title, etc); displays "hidden" ASCII info.
Drag and Drop and "Send To" support, multi-file (batch) processing support.
Copy/Paste or text export GSpot information - format is user configurable.
Built-in database of 350 video and 150 audio codec types.
Advanced UI including "dual-mode" and hyperlinked "persistent" tool tips.
Full support for OGG media files as well as AVI stream formats.
Identification (only) of non-AVI files (.flv, mp4, .mpg, .mov, .qt, .rm, .swf, .wmv, .asf, etc.)
Supports VFW, ACM, DirectShow and DMO codec types (audio and video