Hiring a helper (temporary, adhoc, part-time, full-time or whatnot) is something of mutual trust and respect. Of course, before hiring anyone, it's good that you do a detailed check on the helper beforehand - and letting them interact with you and your family first whenever possible so you can roughly have a feel of the working ethics and personality of this person. When you do select someone to help you, always remember to have mutual trust and respect - after all, the person is here to help you in things you may not have time for, and he/she is still a human being and a son/daughter to someone else. If you decide to hire someone, trust in them to do their jobs well and not try to dictate or force them to do things - teach and guide if you must, but not reprimand or "discipline"! Respect is important too, for the same reason above. Basically, put yourself in their shoes and treat them how you'd like to be treated - treat them like your family, be there when they need someone to talk to or confide in, let them have their bouts of freedom and give them the privacy every human should have. As tragic as it sounds, you can never 100% prevent these things from happening - but you can definitely reduce the chances of it happening :)
I think one should try to understand your Helper and their background before they start working for you. Once you know their background you can ascertain to a 80-90% accuracy if they will cause trouble later on. Also try to treat your helper with respect, which also means letting them have their appropriate off days and access to a phone and internet so they can Contact home if they are feeling home sick
This is a tough question. I believe that the agencies do a background check. In addition, it is up to us to treat them nicely, considering that they are humans. In my experience, niceness is always receprocated.