How do I know if my MP3 is 320 Kbps?

The general rule of thumb goes like this:

1. Cut-off at 11kHz = Bitrate of 64 kbps.
2. Cut-off at 16 kHz = Bitrate of 128 kbps.
3. Cut-off at 19 kHz = Bitrate of 192 kbps.
4. Cut-off at 20 kHz = Bitrate of 320 kbps.
5. Cut-off at 22 kHz = Bitrate of 500 kbps.