There is any way to know (with encspot or any other tool) if an mp3 file has been re-encoded to a higher bitrate?, not in evident cases where the quality could be noticed by hearing, I refer, for example, to an mp3 that has been re-encoded from 224kbps to 320kbps

no not really. re-encoded mp3s should be easy to identify by sound quality. the side effects are watery, squishy sound, a lot of high freqency artifacts. the frequency response would probably be flat 15 kHz.

There still is a bunch of 320 Blade files out there that sound like they were transcoded from 128's.

They can be detected by a spectrum analysis in cool edit or such by the 16 ish lowpass.

They also sound like shit

FWIW I only found a few of these in several years. Just give the files a quick listen. When in doubt download a few other available files for comparison.

oh yeah, i forgot about blade. do people still use this thing?

let's also not forget all the fucking idiots out there who create their own "high quality" custom command line with lame that include just about every possible switch. those too i reckon can account for some of the mp3s that sound like they were transcoded despite being encoded only once.