Today I encoded the same CD track with LAME at various bitrates (96, 128, 192, V0, V1)
and then tried to ABX them. I wanted to find if encoding my CD collection
at 256kb/s was really worth it

I could hear very clear artifacts at 96kbs; i had little initial difficulties
at 128kbs but after 2/3 minutes i was able to ABX it easily. I noticed
a clear hiss just before any high sound (is this "pre-echo"?).

Then I tried to ABX a 192kb/s MP3 and then a V1 and a V0, with little
or no results... What glitches/artifacts should I be looking for?

(i know someone may think this is cheating, but this is
not a competition btw )

Listening initially at lower bit rates to learn what to listen for at higher bit rates is definitely not considered cheating. It is a well established method of training to hear artifacts.

At 192+ kbps mp3 is transparent to most people with nearly every kind of music.
If you encoded at 256 kbps this was not necessary in this sense, but you get a security margin against those rare musical samples where even 192 kbps is not enough. For extremely bad problems however (usually originating from electronic music) you can't get transparency with mp3 (though with very high bitrate quality usually is acceptable even in these extreme cases).

It's a questionable thing if you do want to look for samples that aren't perfect at 192 kbps with a good encoder.
If you're not into electronic music or very special musical instruments you don't have to care.
I'd decide on the bitrate according to whether you're rather a practically minded person or a bit of a perfectionist:
Practically minded people prefer a bitrate of ~ 192 kbps maximum because everything's fine this way in a practical sense. Perfectionists prefer a higher bitrate resp. quality setting.