Based on their actions in prison. If they're constantly in fights, they won't get paroled. If they've become 'model' prisoners and not been disciplinary problems, done work releases, etc., then they will. Once again, it's based on their actions in prison and not some reading on a thought-o-meter.
That's a nice sidestep, but sadly amiss. A large part of these decisions come down to psychological profiling (mind reading, if you will)...and in the case of sentencing, personal opinion based on assumed remorse or lack thereof.
