this is why you are taught to ask permission first, let them know your a qualified first aider, then once they give u permission, whether its getting them to squeeze ur hand, or verbally say something, you would proceed.

I've broken someone's ribs before giving first aid... guy was thankful i saved his life, and not so worried about his ribs (at the time... im sure he cursed me later tho)

I've never heard of anyone being sued in Australia for it, but with the American-isms that are finding their way into our society, it would probably be possible to do it... Who knows, in the times I have done rescues, I didnt have time to think about that.