It depends on the species of bird. Crows caw, chickens squak, ducks quack, etc. You need to be more specific.
You can't until they are a certain age. Before that, while the females quack, the males make a 'whong' or 'woooo' sound. If the bill is yellow-green it is probably a male.
You could also take it to the vet and have it sexed.
It depends on the bird, some people think that a fake owl strategically placed will keep some birds away.