1. Download qaviinfo.exe and save it to the same folder that your AVI file is located. Mine is at C:\Downloads for this example.
2. Open a DOS command prompt (Start > Run > CMD) and navigate to that folder. Type CD C:\Downloads.
3. The name of the video file in my case is presenta.avi. So I can now type qaviinfo presenta.avi.
4. A list of information about the file will be displayed. In the Codec column you can see what codec is being used. Here it is FMP4.