How did shanks know luffy was the one?

Shanks went around to probably hundreds of islands and met and talked to hundreds of kids just like luffy but he decided to trust a devil fruit he was aware of the power near a kid like luffy above that after not only give his straw hat passed down from Rodger to luffy but then even give up his arm to protect luffy and make a promise that luffy would give it back, shanks clearly knew luffy would be the king of the pirates but how was it conquers haki I just don’t understand if someone could explain please go ahead 😭