A quick test would be to toss a hard object at the guy’s head without warning. If he doesn’t react fast enough to catch it, he get’s a bruise and a life sentence. If he catches or deflects it; he avoids a bruising, but stays on death row.
There’s a fairly high correlation between reaction time and IQ as measured by psychometric tests. That’s because both IQ (as we understand it) and RT depend on speed of information processing.
If he gets hit and claims it was bad of you to throw the object then he understands right from wrong.