How to determine the current aspect ratio of an mp4 file

Running Windows Media Center on Windows 7 operating system.

Many mp4 files now play in letterbox mode. They used to play in full screen.

I copied an mp4 file to another Windows 7 pc and the file played in letterbox mode.

So, I have to assume somehow the mp4 file changed.

1.) How can I determine the aspect ratio of an mp4 file?

2.) How does knowing the frame width and frame height help determine aspect ratio?

Answer 1: Install MediaInfo...
...then right click on a media file
...and select MediaInfo from the context menu.

Answer 2: I'll let others explain how the aspect ratio is determined from those numbers.