How to know whether a audio file is CBR or VBR?

Install Checkmate first (the .deb file) by double clicking it and selecting Install in Ubuntu Software Center.

Then, open up a terminal with CtrlAltT and call:

mpck input.mp3 | grep "bitrate"

This will tell you precisely whether a file is CBR or VBR. If it's CBR, you'll just see the bitrate, and if it's VBR, after the average bitrate label you'll see (VBR).

I tested this on Ubuntu 12.04, but packages for Checkmate are available for Windows as well.

This is my trick, it works only if you have a directory containing multiple mp3 files, and you know that they have the same encoding (VBR or CBR): If the files show different bitrates then you know they are VBR encoded.

You see the bitrate indication in the file properties, or use exiftool *.mp3 | grep Bitrate.