Gotta get my fugu on.
As I recall, the first indication your fugu has been improperly prepared is a pins and needles feeling in your fingers and toes. But when it HAS been properly prepared, you'll know because you'll get a pins and needles feeling in your fingers and toes.
Fugu contains a neurotoxin which is not deactivated by heat. So it doesn’t really matter how long you cook it, it’s still lethal.