The Mind Probe was a piece of scanning technology that detects traces of criminal activity from the mind of a person being scanned.
It was designed to find crimes that have been hidden from the law and thus bring the perpetrators to justice. It was standard activity to have those entering penal colonies to undergo it (unless they have a clearance code given before they land).
When the Justice Computer scanned Dave Lister in the episode Justice, it found evidence of petty criminal acts from his adolescence, but let him pass. When it scanned Arnold Rimmer, it detected his guilt for the role he played in the Cadmium II accident that killed the crew of the Red Dwarf and charged him with second degree murder, with 1167 counts - one for each member of the crew except Lister and Rimmer himself.
Noting his hologrammatical status, these many charges were to be served consecutively. He was saved from this fate thanks to the defence by Kryten who pointed out one of the faults of the device. It detected Rimmer's guilt but as Kryten pointed out, he was too subordinate to really be to blame (and that it was the fault of the one who ordered him). The Mind Probe detects guilt, but that does not necessarily mean culpability. The Mind Probe would also seemingly be useless for sociopaths as they do not feel guilt over their crimes. (Justice)