Method 1: Easiest - File name parsing. If the filename matches a known list of video file types (i.e. avi mp4 mpeg etc), then it matches a video file. This is not as robust since I can easily name a text file with the extension .avi, but that does not necessarily make it a video file.
Method 2: Harder - Parse file header. For example, at
CodeProject [
^]there is a C# RIFF Parser