I recently downloaded a relatively high bitrate mp3 file (236kbps) encoded with Lame 3.91, and would like to know whether this is a r3mix or aps encoded file (or maybe some other command line form). Are there any specific differences I could notice with say, a spectrum analyzer/Encspot Pro? Or am I pretty much SOL?

APS uses nspsytune, r3mix not, I think you can see this in the LAME Tag with encspot. If you can see lowpass, I think APS has a higher one.

See here:

possible to detect lame encoding settings in an mp3; part II
and
Lame Tag "Quality Index"

I have to correct Garf:
r3mix uses nspsytune and has a higher lowpass.

True. I think Garf meant that --r3mix doesn't use nssafejoint.

No, I was totally wrong. I remembered that nspsytune has problems without proper tweaks, and r3mix doesn't have those, so I assumed it used gpsycho, my bad.

Too long since I used --r3mix

Thank you all! Turns out it's a --r3mix file, although without the original (and moreover with the not-so-high-quality audio equipment I have at my disposal at the moment), it's not like I'm going to notice a difference anyway.

Actually, it's much easier to go with CiTay's link to the quality index in encspot: --aps(e) always has 78, r3mix 88 (that is, of course, if the LAME tag's there in the first place).