mettere una mano sul fuoco
(to put the hand on the fire) •
It's used when you are absolutely certain of what you are saying. On the contrary, when you are not that sure, you can use it in the negative form.
"Sei sicuro che sia la strada giusta?"
(+)"Certo, ci metterei la mano sul fuoco"
(-) "Credo di si, ma non ci metterei la mano sul fuoco"
"Are you sure this is the right way to go?"
(+)"Sure, I would put my hand on the fire"
(-) "I think so, but I wouldn't put my hand on the fire"