I downloaded some MP3s which were claimed to be 192kbps but to my ears they sound more like 128kbps. I think the guy who sent them to me re-encoded them at a higher bitrate but he won't admit to it. Is there any way I can confirm my suspicions (software)? Thanks!

AFAIK no. the only means are your ears. it could be a very bad encode though, you might want to check wit MrQuestionMan or EncSpot to rule this out.

Well, if you ask the guy for the encoder and the settings he used to encode the files, you will know what low-pass it should have. If it has more frecuences cutted than it should, maybe he is not telling you the truth, and has re-encoded the files with lower settings (and with a lower low-pass filter).

My english is poor, but I hope I have been able to explain myself.

Possibly the source material is just badly produced/mastered. I have no way of knowing unless I can track down the actual CD, which is kind of rare and expensive. Thanks for the info.

128kbps mp3 normally cuts off freq above ~15.5kHz, if there were freq above that, it will be very very rare
U can chack it with spectrum analyzer

192kbps mp3s are normally lowpassed at ~19kHz

bad mp3 encoder like Xing lowpass at ~16kHz no matter what is the bitrate

bad in theory, yes.
but listening tests show it's not quite so bad (at least earlier versions).