I think booger said it first--ask either computer "what would the other computer say is the safe button?" Going by this we know that the lying computer will recommend the opposite of what the other will say and the truthful computer would recommend exactly what the other will say. So if you happen to ask the lying computer what the truthful computer would say, he'd give the opposite and tell you the bomb button. If you asked the truthful computer what the lying computer would say, he'd give the truth. Either way you find out which button is the safe button.
However, if the computers aren't networked then that's a whole different paint job.