If we managed to create computers that had consciousness, how would we know?

Someday it is likely that we will create computers that are capable of responding to any given input in a manner that is indistinguishable from a human response. But, does this make them conscious? I don’t know what exactly makes the distinction, but a computer that has been programmed to have realistic responses feels somehow fundamentally different from a conscious human.
We can’t even really prove that other people are conscious; being unable to get out of our own heads, we basically just assume that other people are conscious like we are. So if we created conscious computers, how would we know it?
Or, is the computer described above conscious by definition? Is the ability to have the “proper” output for any given input the criteria for consciousness? Or is there something indefinably different about human consciousness?