Something I was recently told when looking at grey heads.
When you receive a head or when you remove one from a motor, don't initially scrape any of the carbon away. If there is a crack you should see it easily in the carbon build-up. Between the valves or towards the edge of the combustion chamber. If you have sand blasting equipment give the head a blast and any crack will stick out like you know what.
Another thing; DON'T throw any away unless you have proved it is cracked. There is ALWAYS someone who needs one or someone who will take it off your hands.
Colin.