my grammar book has the following explanation on the usage of "to infinitive"
You will take him for an American to hear him speak English.
= You will take him for an American if you hear him speak English.
Never have I come across with such usage during my 8-year stay in nz, is it a common usage? isn't the meaning of "to hear" a little bit ambiguous even to the natives? or do you guys understand the first sentence as clearly as the second one?