Don't doubt your ears.
Make 128 bitrate MP3s and compare them to the source wavs.
You will hear the difference on a good system.
Now try it with 192 bitrate MP3s.
Once you know what to listen for from the lower speed samples, the higher speed ones become more discernable.
Spectrum, punch and attack, clarity and accuracy of HF, and that damnable phasiness is what I hear as big differences.
Verify ? Not 100% without knowing the processing chain. There is no header that passes on a file which shows that it is decoded from an MP3 to my knowledge.
It's all about the sound.